Multi-beam exposure unit

ABSTRACT

This invention has as its object to provide an image forming apparatus which can provide a color image free from any color misregistration with low cost, and a light scanning unit suitable for the apparatus. The apparatus includes a finite lens and a cylinder lens for giving predetermined characteristics to light beams emitted by a plurality of light sources, a laser synthesis mirror unit for synthesizing laser beams passing through the lenses, a deflection unit for deflecting the synthesized light beams, first and second image-forming lenses for adjusting the aberration characteristics, at the imaging position, of the deflected light beam, one or three mirrors for outputting the light beam passing through the image-forming lenses at predetermined positions, and can provide a color image free from any color misregistration.

This application is a divisional or of application Ser. No. 08/780,905,filed Jan. 9, 1997, U.S. Pat. No. 5,751,462.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-beam laser exposure suitablefor an image forming apparatus such as a color printer apparatus, a highspeed copying machine and a color copying machine, each having pluralityof drums.

For example, an image forming apparatus such as a color printer, a colorcopying machine, or the like, each having a plurality of drums, uses aplurality of image forming units for forming images corresponding tocolor-separated color components, and a laser exposure unit forproviding a plurality of image data, i.e., a plurality of laser beams inunits of color components to these image forming units. As the laserexposure unit, for example, a plurality of laser exposure units arearranged, or a multi-beam laser exposure unit which can produce aplurality of laser beams is arranged.

The multi-beam laser exposure has a semiconductor laser element servingas a light source, a first lens group for focusing the beam size of alaser beam emitted by the laser element to a predetermined size, a lightdeflection unit for continuously reflecting the laser beam focused bythe first lens group in a direction perpendicular to the feed directionof a recording medium, a second lens group for imaging the laser beamdeflected by the light deflection unit at a predetermined position ofthe recording medium, and the like. In general, the direction in whichthe laser beam is deflected by the light deflection unit is called amain scanning direction, and the direction in which the recording mediumis rotated, i.e., the direction perpendicular to the main scanningdirection, is called a sub-scanning direction.

As a light scanning unit of this type, for example, depending on theimage forming apparatus to which the light scanning unit is applied, aplurality of light scanning units are arranged in correspondence withthe image forming units or a multi-beam light scanning unit which canproduce a plurality of laser beams is arranged.

If image information can be recorded on a recording medium using, e.g.,N laser beams, the rotational speed of a rotary mirror and the imagefrequency can be reduced to 1/N.

By arranging M groups of light sources including N laser beams incorrespondence with the number of color-separated color components, acompact light scanning unit suitable for an image forming apparatus thatcan form a color image can be provided.

However, in order to guide M groups of laser beams to the lightdeflection unit in a state wherein they can be considered as a singlelaser beam, the M groups of laser beams must be synthesized on the lightsource side of the light deflection unit. In this case, a sufficientlylarge distance must be assured between the light deflection unit and thelight source, or the laser beams incident on the reflection surface ofthe light deflection unit must be incident to be separated from eachother in the direction perpendicular to the rotation direction of thereflection surface, i.e., the sub-scanning direction.

When a large distance is assured between the light source and the lightdeflection unit, the light scanning unit becomes large in size. On theother hand, when the laser beams are incident on the reflection surfaceto be separated from each other in the sub-scanning direction, theimaging characteristics may deteriorate, or the bend amount differencesin the scanning direction in units of M groups of laser beams,variations in sectional beam size on the image surface upon changes inrefractive index of the lens material caused by environmental changes inunits of M groups of laser beams, and the like may increase.

If the distance between the final lens surface and the image surface isincreased for the purpose of attaining a size reduction of the lightscanning unit, the bend amount differences in the scanning direction inunits of M groups of laser beams increase. In this case, the bend amountcan be reduced by increasing the distance between the reflection surfaceof the light deflection unit and the final lens surface, but the size ofthe light scanning unit increases. On the other hand, when the distancebetween the reflection surface and the final lens surface is increased,the driving frequency upon driving the laser element of each lightsource, i.e., the image frequency must be raised since the effectivescanning angle decreases. As a consequence, cost increases in terms ofnoise measures and the frequency characteristics of the driving unit.

Furthermore, upon optimizing the lens passing positions of the M groupsof laser beams separated in the sub-scanning direction to obtain uniformoptical characteristics to be given to these laser beams, the laserbeams that pass through positions offset in the sub-scanning directionfrom the optical axis of the system of the light scanning unit maysuffer coma different from that for the remaining laser beams.

When each of the M groups of laser beams includes N laser beams, aplurality of half mirrors as semi-transparent mirrors are used tosynthesize the N laser beams so that they are substantially consideredas a single laser beam. In this case, when the laser beams pass throughdifferent numbers of half mirrors, the light amount difference,spherical aberration difference, coma difference, and the like among thelaser beams increase, resulting in different beam sizes.

Furthermore, when each of the M groups of laser beams includes N laserbeams, since each group of laser beams has a width in the sub-scanningdirection, the tilt, in the main scanning direction, between theexposure start and end positions of laser beams scanned in a single scanmay increase up to a visible level.

Moreover, since each of the M groups of laser beams includes N laserbeams, the optical energy on the image surface may vary due to the phasedifference or wavelength variations of the laser beams when the lightintensities of all the laser beams that have reached the image surfaceare synthesized. When variations in optical energy due to the phasedifference or wavelength variations have exceeded a predeterminedamount, the image may be locally lost or toner supply to an unexposedportion may be lost when such unit is built in the image formingapparatus.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light scanningunit that can form a color image suffering less color misregistration.

According to a first aspect of the present invention, there is providedan optical exposure unit comprising: light sources which are arranged incorrespondence with numbers indicated by N₁ to N_(M) (M is an integernot less than 1) and emit light beams; first lens means for convertingthe light beams emitted by each of the light sources into one ofconvergent light and collimated light, the lens means including one of afinite lens and collimate lens in number corresponding to a sum of N₁ toN_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans including at least one lens, and imaging each of the light beamsdeflected at an equal speed by the deflection means at a predeterminedposition, and wherein M beam groups are incident on the reflectionsurface of the deflection means so that an interval between adjacentbeam groups monotonously increases from one end, and a beam group on oneend with a smallest interval between adjacent beam groups is incident tocross the beams deflected by the deflection means.

According to the second aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans including at least one lens, and imaging each of the light beamsdeflected at an equal speed by the deflection means at a predeterminedposition, and wherein a distance L₀ between a final lens surface and animage surface in a light scanning unit for irradiating M beam groupsonto M image carriers falls within a range defined by:

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/1.8>L.sub.0

    L.sub.0 >(ΔL.sub.MAX +L.sub.M +L.sub.1)/2

where L₁ is the distance between an optical axis of a system of secondoptical means and a scanning line on one end, L_(M) is the distancebetween the optical axis of the system of the second optical means and ascanning line on the other end, and ΔL_(MAX) is the distance, in adirection parallel to the optical axis of the system, between thescanning lines at the two ends.

According to the third aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans including at least one lens, and imaging each of the light beamsdeflected at an equal speed by the deflection means at a predeterminedposition, and wherein an effective field angle φ of a beam deflected bythe deflection means satisfies:

φ>W/L_(t) where L_(t) is the distance between a reflection point on thedeflection means and an image surface, and W is the effective imageregion width including a region where a horizontal synchronizationsignal is detected.

According to the fourth aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans including at least one lens, and imaging each of the light beamsdeflected at an equal speed by the deflection means at a predeterminedposition, and wherein an incident angle of a beam to M sets of opticalmembers given positive power in only a sub-scanning direction has apredetermined tilt, and a position of the beam is decentered from anoptical axis of each of the optical members.

According to the fifth aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and secondoptical means including a lens having a function of imaging N_(i) beamsdeflected by the deflection means to be scanned on a predetermined imagesurface at an equal speed, and correcting a surface inclination of thedeflection means, wherein a scanning line is tilted from a directionperpendicular to a traveling direction of an image carrier by an angle:

δ=tan⁻¹ (N_(i) ×p×k×φ/(4×π×W)) where p is the scanning pitch in asub-scanning direction, and k is the number of rotary polygonal mirrorsurfaces.

According to the sixth aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and secondoptical means including a lens having a function of imaging N_(i) beamsdeflected by the deflection means to be scanned on a predetermined imagesurface at an equal speed, and correcting a surface inclination of thedeflection means, wherein the unit satisfies: ##EQU1## where p is thebeam pitch in a sub-scanning direction, β is the e⁻² diameter, in a mainscanning direction, of a beam/p, α is the e⁻² diameter, in thesub-scanning direction, of the beam/p, ζ is the half exposure amount ofa photosensitive body/average exposure energy, and η is the intensity ofone beam relative to the peak intensity of the other beam at the middlepoint of a line connecting centers of two neighboring beams.

According to the seventh aspect of the present invention, there isprovided an optical exposure unit comprising: light sources which arearranged in correspondence with numbers indicated by N₁ to N_(M) (M isan integer not less than 1) and emit light beams; first lens means forconverting the light beams emitted by each of the light sources into oneof convergent light and collimated light, the lens means including oneof a finite lens and collimate lens in number corresponding to a sum ofN₁ to N_(M) ; second lens means given lens power associated with a firstdirection to converge the light beams output from each of the lens meansin only the first direction, the M sets of the second lens means beingprepared; deflection means for deflecting the light beams output fromthe second lens means in a second direction perpendicular to the firstdirection, the deflection means including a reflection surface which isformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans including at least one lens, and imaging each of the light beamsdeflected at an equal speed by the deflection means at a predeterminedposition, and wherein M sets of optical members given positive power inonly a sub-scanning direction includes a one-sided cylinder lensconsisting of glass, and a double-sided cylinder lens substantiallyequivalent to a material of a post-deflection optical system lens.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic sectional view of an image forming apparatus whichuses a multi-beam laser exposure according to an embodiment of thepresent invention;

FIG. 2 is a schematic plan view showing the layout of optical members ofa light scanning unit built in the image forming apparatus shown in FIG.1;

FIG. 3 is a partial sectional view of a pre-deflection optical system ofthe light scanning unit shown in FIG. 2 taken along the optical axis ofthe system between a first light source and a light deflection unit;

FIG. 4 is a diagram showing the relationship among the effectiveimage-angle of the beam, the width of the effective image region, thedistance between the reflection point and the image surface and theoptical performance of the apparatus;

FIG. 5 is a partial sectional view, in the sub-scanning direction, ofthe light scanning unit in FIG. 2 to show the states of first to fourthlaser beams propagating toward the light deflection unit;

FIG. 6 is a schematic sectional view of the optical scanning unit shownin FIG. 2 taken along at the position where the deflection angle of thelight deflection unit 0°;

FIGS. 7A and 7B are schematic plan views showing the layout states ofthe optical members of the pre-deflection optical system of the lightscanning unit shown in FIG. 2;

FIGS. 8A and 8B are respectively a plan view and a side viewillustrating a laser synthesis mirror unit of the light scanning unitshown in FIG. 2;

FIGS. 9A and 9B are schematic views depicting the region wherephotosensitive drums built in the image forming apparatus can bearranged when the light scanning unit shown in FIG. 2 is used;

FIGS. 10A and 10B are schematic views showing the method of defining themountable region of the process-related members used in the scanningunit shown in FIG. 2;

FIG. 11 is a graph obtained by plotting the area which the smaller ofthe regions S₁ and S₂, both shown in FIG. 10, has when ζ=1.4, L₂ =175,λ=0.00068 and ω₀ =0.025;

FIG. 12 is a graph obtained by plotting the value which the equation(a-14) yields when λ=0.00063;

FIG. 13 is a graph obtained by plotting the value which the equation(a-14) yields when λ=0.0008 regarded as a practical value;

FIGS. 14A and 14B are schematic views illustrating the positionalrelationship which laser beams on the image surface have when thescanning unit shown in FIG. 2 is used;

FIG. 15 is a schematic perspective view of a mirror for horizontalsynchronization detection in the light scanning unit shown in FIG. 2;

FIG. 16 is a schematic perspective view showing the adjustment mechanismof an exit mirror in the light scanning unit shown in FIG. 2;

FIGS. 17A to 17C are schematic views showing the beam position of laserbeams irradiated onto the photosensitive drum by the light scanningunit;

FIGS. 18A and 18B are graphs for explaining the relationship between thephase difference and the intensity distribution of laser beamsirradiated onto the photosensitive drum;

FIGS. 19A and 19B are graphs showing the normalization results of theintensity distributions shown in FIGS. 18A and 18B by average exposureenergy;

FIGS. 20A and 20B are graphs showing the relationship between the phasedifference and the intensity distribution of laser beams irradiated ontothe photosensitive drum by the light scanning unit shown in therespective drawings under the same conditions as in FIGS. 18A and 18B;

FIGS. 21A and 21B are graphs showing the normalization result of theintensity distributions shown in FIG. 20A and 20B by average exposureenergy using the same method as in FIGS. 19A and 19B;

FIG. 22 is a graph showing a setting example of elements of the lightscanning unit that can remove the influence of the phase differencebetween neighboring laser beams upon using a group of laser beamscombining at least two laser beams; and

FIG. 23 is a block diagram of a control unit of the image formingapparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention will be describedhereinafter with reference to the accompanying drawings.

FIG. 1 is a front sectional view of a four-drum type color image formingapparatus. Since a multi-color laser beam printer apparatus uses fourdifferent image data color-separated in units of color components ofYellow=Y, Magenta=M, Cyan=C, and Black=B, and four sets of various unitsfor forming images in units of color components in correspondence withY, M, C, and B, the image data in units of color components and thecorresponding units are identified by appending Y, M, C, and B tocorresponding reference numerals.

An image forming apparatus 100 has first to fourth image forming units50Y, 50M, 50C, and 50B for forming images in units of color-separatedcolor components, i.e., Y=yellow, M=magenta, C=cyan, and B=black.

The respective image forming units 50(Y, M, C, and B) are seriallyarranged underneath a light scanning unit 1 in the order of 50Y, 50M,50C, and 50B in correspondence with the exit positions of laser beamsL(Y, M, C, and B) corresponding to the respective color component imagesvia third mirrors 37Y, 37M, and 37C, and a first mirror 33B of the lightscanning unit 1.

A conveyor belt 52 for conveying images formed by the image formingunits 50(Y, M, C, and B) is arranged below the image forming units 50(Y,M, C, and B).

The conveyor belt 52 is looped between a belt driving roller 56 rotatedin the direction of an arrow by a motor (not shown) and a tension roller54, and is rotated in the rotation direction of the belt driving roller56 at a predetermined speed.

The image forming units 50(Y, M, C, and B) respectively havephotosensitive bodies 58Y, 58M, 58C, and 58B, each of which has acylindrical drum shape and is rotatable in the direction of an arrow,and on each of which an electrostatic latent image corresponding to animage is formed.

Around the photosensitive bodies 58(Y, M, C, and B), chargers 60Y, 60M,60C, and 60B for giving predetermined potentials to the surfaces of thephotosensitive bodies 58(Y, M, C, and B); developing units 62Y, 62M,62C, and 62B for developing an electrostatic latent image formed on thesurface of each of the photosensitive bodies with corresponding colortoners; transfer units 64Y, 64M, 64C, and 64B which oppose thephotosensitive bodies while the conveyor belt 52 is inserted betweeneach of the photosensitive bodies and themselves, and transfer tonerimages on the photosensitive bodies onto the conveyor belt 52 or arecording medium conveyed via the conveyor belt 52; cleaners 66Y, 66M,66C, and 66B for removing the residual toners on the photosensitivebodies after the transfer units 64(Y, M, C, and B) transfer the tonerimages; and charge removers (charge removing lamps) 68Y, 68M, 68C, and68B for removing the residual potentials on the photosensitive bodies 58after the transfer units 64(Y, M, C, and B) transfer the toner images,are arranged in turn along the rotation direction of each of thephotosensitive bodies.

Note that each of laser beams LY, LM, LC, and LB guided to thephotosensitive bodies (drums) 58 by the mirrors 37Y, 37M, 37C, and 33Bof the light scanning unit 1 is split into N_(i) beams in thesub-scanning direction above the corresponding photosensitive body, andthe N_(i) beams are irradiated between the chargers 60(Y, M, C, and B)and the developing units 62(Y, M, C, and B), as will be described laterwith reference to FIGS. 2 to 6. In this example, each of the laser beamsLY, LM, and LC is split into two beams (N₁ =N₂ =N₃ =2), and the laserbeam LB is split into four beams (N₄ =4).

A paper cassette 70 is arranged beneath the conveyor belt 52. The papercassette 70 stores recording media, i.e., paper sheets P, on each ofwhich images formed by the image forming units 50(Y, M, C, and B) are tobe transferred.

A pick-up roller 72 is arranged on one end portion of the paper cassette70 and at the side in the vicinity of the tension roller 54. The pick-uproller 72 picks up the paper sheets P stored in the paper cassette 70one by one in turn from the uppermost one and has a semi-circularsection.

Registration rollers 74 are arranged between the pick-up roller 72 andthe tension roller 54. The registration rollers 74 align the leading endof one paper sheet P picked up from the cassette 70 with the leading endof a toner image formed on the photosensitive body 58B, by the imageforming unit 50B.

A chucking roller 76 is arranged between the registration rollers 74 andthe image forming unit 50Y and in the vicinity of the belt drivingroller 56, i.e., substantially on the outer circumferential surface ofthe tension roller 54 to sandwich the conveyor belt 52 therebetween. Thechucking roller 76 provides a predetermined electrostatic chucking forceto one paper sheet P conveyed at a predetermined timing via theregistration rollers 74. Note that the axis of the chucking roller 76 isparallel to that of the tension roller 54.

Registration sensors 78 and 80 for detecting the position of an imageformed on the conveyor belt 52 or a sheet-like transfer medium Tconveyed by the conveyor belt are arranged in the vicinity of the beltdriving roller 56, i.e., substantially on the outer circumferentialsurface of the belt driving roller 56 to sandwich the conveyor belt 52therebetween, so as to be separated from each other by a predetermineddistance in the axial direction of the belt driving roller 56 (SinceFIG. 1 is a front sectional view, only the rear sensor 80 isillustrated).

A conveyor belt cleaner 82 for removing toner attached onto the conveyorbelt 52 or paper dust of the paper sheets P is arranged on the conveyorbelt 52 at a position corresponding to the outer circumferential surfaceof the belt driving roller 56.

A fixing unit 84 for fixing toner images transferred onto a paper sheetP thereto is arranged in a direction in which the paper sheet P conveyedvia the conveyor belt 52 is released from the tension roller 56 and isfurther conveyed.

FIG. 2 is a schematic plan view of the multi-beam light scanning unitused in the image forming apparatus shown in FIG. 1.

As shown in FIG. 1, the multi-beam light scanning unit 1 has a lightdeflection unit 5 for deflecting a laser beam emitted by a laser elementserving as a light source toward the image surface arranged at apredetermined position, i.e., predetermined positions of thephotosensitive drums 58(Y, M, C, and B) of the first to fourth imageforming units 50 shown in FIG. 1 at a predetermined linear velocity.Note that the direction in which the laser beam is deflected by eachreflection surface of the light deflection unit 5 will be referred to asa main scanning direction hereinafter.

The light deflection unit 5 has a polygonal mirror main body 5a on whicha plurality of (e.g., eight) plane reflection mirrors (surfaces) arearranged in a polygonal pattern, and a motor (not shown) for rotatingthe polygonal mirror main body 5a at a predetermined speed in the mainscanning direction.

The polygonal mirror main body 5a consists of, e.g., aluminum.

Each reflection surface of the polygonal mirror main body 5a is providedby cutting a material along a plane including the rotation direction ofthe polygonal mirror main body 5a, i.e., a plane perpendicular to themain scanning direction, that is, the sub-scanning direction, anddepositing a surface protection layer of, e.g., SiO₂ on the cut surface.

A post-deflection optical system 21 is arranged between the lightdeflection unit 5 and the image surface. The post-deflection opticalsystem 21 gives predetermined optical characteristics to a laser beamdeflected in the predetermined direction by each reflection surface ofthe light deflection unit 5.

The post-deflection optical system 21 includes two, i.e, first andsecond lenses 30a and 30b, and guides a laser beam reflected in thepredetermined direction by each reflection surface of the lightdeflection unit 5 to the predetermined position of each of thephotosensitive drums 58(Y, M, C, and B).

A horizontal synchronization detector 23 is arranged at the position ofthe second image-forming lens 30b of the post-deflection optical system21, i.e., at a position corresponding to a predetermined position beforea position corresponding to the write start position, on each of thephotosensitive drums 58, of an image of each of the laser beams L(Y, M,C, and B) emerging from the second image-forming lens 30b toward thephotosensitive drum 58 in the rotation direction of the reflectionsurface of the light deflection unit 5.

A horizontal synchronization detection mirror 25 is inserted between thepost-deflection optical system 21 and the horizontal synchronizationdetector 23. The mirror 25 reflects some light components of the laserbeams L(Y, M, C, and B), which have passed through at least one lens(30a or 30b) included in the post-deflection optical system 21 and areobtained by synthesizing 4×2 beams toward the horizontal synchronizationdetector 23 in different directions in both the main scanning andsub-scanning directions.

A pre-deflection optical system between the laser element serving as thelight source and the light deflection unit 5 will be described in detailbelow.

The light scanning unit 1 has yellow, magenta, and cyan light sources3Y, 3M, and 3C each including two, i.e., first and second laser elements(N₁ =N₂ =N₃ =2) that satisfy N_(i) (i is a positive integer), and afourth light source 3B including four, i.e., first to fourth laserelements (N₄ =4) that satisfy N_(i) (i is a positive integer). Note thatthe number M (M is a positive integer) of beam group light sources is 4.

The first light source 3Y has first and second yellow laser elements 3Yaand 3Yb for emitting laser beams corresponding to Y, i.e., a yellowimage. These laser elements 3Ya and 3Yb are arranged to be able to set apredetermined distance in the sub-scanning direction between laser beamsLYa and LYb emitted thereby on each reflection surface of the polygonalmirror main body 5a of the light deflection unit 5.

The second light source 3M has first and second magenta laser elements3Ma and 3Mb for emitting laser beams corresponding to M, i.e., a magentaimage. Note that these laser elements 3Ma and 3Mb are also arranged tobe able to set a predetermined distance in the sub-scanning directionbetween laser beams LMa and LMb emitted thereby on each reflectionsurface of the polygonal mirror main body 5a of the light deflectionunit 5.

The third light source 3C has first and second cyan laser elements 3Caand 3Cb for emitting laser beams corresponding to C, i.e., a cyan image.Note that this light source 3C is also arranged to be able to set apredetermined distance in the sub-scanning direction between laser beamsLCa and LCb emitted by its lasers on each reflection surface of thepolygonal mirror main body 5a of the light deflection unit 5.

The fourth light source 3B has first, second, third, and fourth blacklaser elements 3Ba, 3Bb, 3Bc, and 3Bd corresponding to B, i.e., a blackimage. Note that the light source 3B includes four lasers and isarranged to be able to set a predetermined distance in the sub-scanningdirection between adjacent ones of laser beams LBa, LBb, LBc, and LBdemitted from its. lasers on each reflection surface of the polygonalmirror main body 5a of the light deflection unit 5 as in the first tothird light sources 3Y, 3M, and 3C.

With this arrangement, M sets (M=4) of N_(i) laser beams, i.e., laserbeams LYa, LYb, LMa, LMb, LCa, LCb, LBa, LBb, LBc, and LBd, which areseparated by the predetermined distance in the sub-scanning direction oneach reflection surface are incident on the reflection surface of thepolygonal mirror main body 5a of the light deflection unit 5.

Four (M) sets of pre-deflection optical systems 7(Y, M, C, and B) arearranged between the laser elements 3Ya, 3Yb, 3Ma, 3Mb, 3Ca, 3Cb, 3Ba,3Bb, 3Bc, and 3Bd, i.e., the four light sources 3Y, 3M, 3C, and 3B, andthe light deflection unit 5. Each pre-deflection optical system 7adjusts, to a predetermined shape, the beam spot sectional shape of eachof 2+2+2 +4 laser beams, i.e., a total of 10 laser beams LYa, LYb, LMa,LMb, LCa, LCb, LBa, LBb, LBc, and LBd emitted by these light sources.

Finite lenses 9Ya and 9Yb for giving predetermined convergence to thelaser elements 3Ya and 3Yb and stops 10Ya and 10Yb for adjusting thebeam sectional shapes of laser beams passing through the correspondingfinite lenses to a predetermined shape are respectively interposedbetween the first and second yellow laser elements 3Ya and 3Yb, and thelight deflection unit 5. Note that the finite lenses 9Ya and 9Yb musthave optical characteristics complementary to those to be given to animage-forming lens group used in the post-deflection optical system (tobe described later), and may adopt collimator lenses each for convertinga laser beam into collimated light depending on the opticalcharacteristics of the image-forming lens group.

A half mirror 12Y as a semi-transparent mirror is arranged at thecrossing position of the laser beams LYa and LYb that have passedthrough the stops 10Ya and 10Yb. The half mirror 12Y superposes thelaser beams LYb and LYa into a beam that may be substantially consideredas a single laser beam LY when viewed from the sub-scanning direction.More specifically, the surface, opposite to the surface that receivesthe laser beam LYa, of the half mirror 12Y receives the laser beam LYb,which is separated from the laser beam LYa by a predetermined beaminterval in the sub-scanning direction. Note that the half mirror 12Y isarranged at a predetermined angle so that the laser beams LYa and LYb,which are superposed on each other into a beam that may be substantiallyconsidered as a single laser beam LY when viewed from the sub-scanningdirection, can be incident on the polygonal mirror main body 5a of thelight deflection unit 5.

A cylinder lens 11Y and a laser synthesis mirror unit 13 are arrangedbetween the half mirror 12Y and the light deflection unit 5. Thecylinder lens 11Y further converges the laser beam LY superposed by thehalf mirror 12Y in only the sub-scanning direction. The laser synthesismirror unit 13 has a plurality of reflection surfaces for guiding thelaser beam LY that has passed through the cylinder lens to the lightdeflection unit 5 substantially as a bundle of light rays, as will bedescribed in detail later with reference to FIGS. 8A and 8B.

The laser synthesis mirror unit 13 also guides other laser beams to bedescribed below substantially as a bundle of light rays to the lightdeflection unit 5. As can be apparent from FIG. 2, the first and secondyellow laser beams LYa and LYb emitted by the first light source 3Y passthrough the laser synthesis mirror unit 13 shown in FIGS. 8A and 8B andare guided to the light deflection unit 5.

Finite lenses 9Ma and 9Mb and stops 10Ma and 10Mb, which respectivelycorrespond to the first and second magenta laser elements 3Ma and 3Mb, ahalf mirror 12M, and a cylinder lens 11M are interposed between thelaser elements 3Ma and 3Mb, and the laser synthesis mirror unit 13.

Likewise, finite lenses 9Ca and 9Cb and stops 10Ca and 10Cb, whichrespectively correspond to the first and second cyan laser elements 3Caand 3Cb, a half mirror 12C, and a cylinder lens 11C are arranged betweenthe laser elements 3Ca and 3Cb, and the laser synthesis mirror unit 13.

Furthermore, first to fourth finite lenses 9Ba, 9Bb, 9Bc, and 9Bd thatcan give optical characteristics similar to the above-mentioned lightsources, first to fourth stops 10Ba, 10Bb, 10Bc, and 10Bd, half mirrors12B₁, 12B₂, 12B₃, and a cylinder lens 11B are inserted between thefirst, second, third, and fourth black laser elements 3Ba, 3Bb, 3Bc, and3Bd, and the laser synthesis mirror unit 13.

Note that the light sources 3(Y, M, C, and B), the pre-deflectionoptical systems 7(Y, M, C, and B), and the laser synthesis mirror unit13 are integrally held by a holding member (not shown) consisting of,e.g., an aluminum alloy.

The optical characteristics of the lenses and half mirrors used in thepre-deflection optical systems will be explained in detail below.

Each of the finite lenses 9(Y, M, C, and B)a, 9(Y, M, C, and B)b, 9Bc,and 9Bd has a single lens obtained by adhering an ultraviolet-settingplastic aspherical surface lens (not shown) to the surface of anaspherical or spherical surface glass lens.

In order to set all the outputs of the laser elements in the respectivebeam groups to be identical values and to obtain identical lightintensity on the surface, the ratios of reflectance to transmittance ofthe half mirrors 12(Y, M, and C) and the half mirror 12B₁ arerespectively set to be 1:1. In contrast to this, the ratios ofreflectance to transmittance of the half mirrors 12B₂ and 12B₃ arerespectively set to be 2:1 and 3:1.

More specifically, since each of the first to third light sources 3Y,3M, and 3C has two lasers (N₁ =N₂ =N₃ =2), the required total number ofhalf mirrors 12(Y, M, and C) is managed by N_(i) -1, and each halfmirror must synthesize 50% each of the light amounts of the laser beamsfrom the two light sources. Therefore, the ratio of reflectance totransmittance is set to be 1:1, and when the laser beams have passedthrough the half mirrors 12(Y, M, and C), the light intensities of thelaser beams LYa, LYb, LMa, LMb, LCa, and LCb emitted by the laserelements 3Ya, 3Yb, 3Ma, 3Mb, 3Ca, and 3Cb are controlled to besubstantially equal to each other.

On the other hand, the half mirror 12B₁ which synthesizes the laserbeams LBa and LBb from the first and second laser elements 3Ba and 3Bbhas the same reflectance and transmittance as those of the half mirrors12(Y, M, and C) since it synthesizes 50% each of the light amounts ofthe laser beams from the two light sources. In contrast to this, sincethe half mirror 12B₂ synthesizes the laser beam LBa that has alreadybeen synthesized by the half mirror 12B₁ with the laser beam LBc fromthe third black laser element 3Bc, the light intensities of the laserbeams LBa, LBb, and LBc can be set to be equal to each other when theratio of reflectance to transmittance is set to be 2:1. Similarly, sincethe half mirror 12B₃ synthesizes the laser beam LBc that has alreadybeen synthesized by the half mirror 12B₂ with the laser beam LBd fromthe fourth black laser element 3Bd, the light intensities of the laserbeams LBa, LBb, LBc, and LBd emitted by the laser elements 3Ba, 3Bb,3Bc, and 3Bd can be set to be equal to each other by setting the ratioof reflectance to transmittance to be 3:1, when the laser beams havepassed through the half mirror 12B₃.

The light intensities of the laser beams LBa, LBb, LBc, and LBd enteringthe hybrid cylinder lens 11B decrease to about 25% as compared to thosewhen they are emitted by the laser elements 3Ba, 3Bb, 3Bc, and 3Bd.

In contrast to this, the light intensities of the laser beams LYa, LYb,LMa, LMb, LCa, and LCb that enter the hybrid lenses 11(Y, M, and C) viathe half mirrors 12(Y, M, and C) are held to be 50% as compared to thosewhen they are emitted by the laser elements 3Ya, 3Yb, 3Ma, 3Mb, 3Ca, and3Cb.

Therefore, in the example shown in FIG. 2, when the rated outputs of thelaser elements 3Ya, 3Yb, 3Ma, 3Mb, 3Ca, and 3Cb are set to be 10milliwatts (to be abbreviated as mW hereinafter), and the rated outputsof the laser elements 3Ba, 3Bb, 3Bc, and 3Bd are set to be 20 mW,different numbers of laser beams for different colors can be set to havesubstantially equal light intensities at the imaging position.

As is known, the optimal light intensity of a laser beam on the imagesurface varies in association with variations of the characteristics oftoners used in the developing units 62(Y, M, C, and B) and/or errors ofthe characteristics of the individual photosensitive drums 58(Y, M, C,and B) used in the image forming units 50(Y, M, C, and B). On the otherhand, toners corresponding to the respective color components are oftenrequired to use different light intensities and beam sizes owing to thecharacteristics of their coloring agents, transfer methods, and thelike. However, the light intensities and beam sizes in the image formingunit corresponding to one color component must be substantially uniform.

More specifically, the light intensities of the laser beam groups LY,LM, LC, and LB need not be uniform among the image forming units 50(Y,M, C, and B), but the N_(i) beams in each beam group must have uniformlight intensities and beam sizes on the image surface.

For example, if the yellow laser beams LYa and LYb have equal lightintensities but the beam size of the beam LYa is smaller than that ofthe beam LYb, the width of a latent image written on the photosensitivedrum 58Y by the yellow laser beam LYa becomes smaller than that of alatent image written on the photo-sensitive drum 58Y by the yellow laserbeam LYb. For example, when lines are to be written in the main scanningdirection at every third line positions, the thicknesses of the linesvary, resulting in a non-uniform image.

In this way, the yellow laser beams LYa and LYb, magenta laser beams LMaand LMb, cyan laser beams LCa and LCb, and black laser beams LBa, LBb,LBc, and LBd are required to have equal light intensities and uniformbeam sizes on the image surface.

For these reasons, in the embodiment of the present invention, the ratedoutputs of the laser elements 3Ya and 3Yb that emit the laser beams LYaand LYb synthesized in the main scanning direction by the half mirror12Y are set to be 10 mW. Also, the rated outputs of the laser elements3Ma and 3Mb that emit the laser beams LMa and LMb synthesized in themain scanning direction by the half mirror 12M are set to be 10 mW.Likewise, the rated outputs of the laser elements 3Ca and 3Cb that emitthe laser beams LCa and LCb synthesized in the main scanning directionby the half mirror 12C are set to be 10 mW. Note that the rated outputsof the laser elements 3Ba, 3Bb, 3Bc, and 3Bd that emit the laser beamsLBa, LBb, LBc, and LBd synthesized by the half mirrors 12B₁, 12B₂, and12B₃ are set to be 20 mW.

The beam sizes of the laser beams in the beam groups, i.e., the beamsizes on the image surface of the laser beams LYa and LYb, LMa and LMb,and LCa and LCa in the beam groups can be easily uniformed usingidentical finite lenses and stops.

In the optical apparatus shown in FIG. 2, if the ratios of reflectanceto transmittance of all the half mirrors 12B₁, 12B₂, and 12B₃ are set tobe 1:1, in order to obtain uniform light intensities on the imagesurface, the rated outputs of the laser elements 3Ba, 3Bb, 3Bc, and 3Bdmust be respectively set to be 40 mW, 40 mW, 20 mW, and 10 mW. In thiscase, the beam sizes vary on the image surface due to theabove-mentioned radiation angle differences of the laser beams.

In this case, it is easy to hit upon a method of using laser elementshaving identical rated outputs, i.e., 40 mW as the laser elements 3Ba,3Bb, 3Bc, and 3Bd, and controlling the outputs in actual use to be 40mW, 40 mW, 20 mW, and 10 mW. However, with this method, the laserelements 3Bc and 3Bd apparently have over-specifications, resulting inan increase in cost.

The number of half mirrors 12 prepared for each of the laser beams LY,LM, LC, and LB is N_(i) -1 (1 for yellow, magenta, and cyan, and 3 forblack) of the number N of beams that constitute each of the M sets oflight sources. However, the number of times of passing through the halfmirror 12 is limited to a maximum of unity independently of the laserbeams.

In other words, the laser beams LYb, LMa, LCb, and LBa are merelyreflected by the half mirrors 12 (the number of times of passing =0),and the remaining laser beams pass through the half mirrors 12 onlyonce.

As described above, by minimizing the number of times of passing of thelaser beams via the half mirrors 12 and the differences in the numbersof times of passing among beams, variations in focal length or theinfluence of spherical aberration as problems posed when light otherthan collimated light passes through a plane-parallel plate can bereduced. When each of the laser beams LYb, LMa, LCb, and LBa which donot pass through any half mirrors 12 passes through plane-parallelplates with a refractive index equal to that of the half mirrors 12,variations in optical characteristics caused by the differences in thenumber of times of passing among beams can be reduced.

Tables 1 to 3 below summarize optical numerical value data of thepre-deflection optical system 7.

                  TABLE 1                                                         ______________________________________                                        Pre-deflection optical system lens data                                       (Unit of angle: rad, unit of length: mm)                                      Effective field angle φ: 1.01237                                          Radius of inscribed circuit of reflection surface of light                    deflection unit: 33                                                           Separation angle: 0.6981317                                                   Center of rotation of reflection surface of light deflection                  unit: (26.28,20.02)                                                           ______________________________________                                        For magenta                                                                   Curvature                                                                     CUY   CUX        Thickness Material                                                                             Remarks                                     ______________________________________                                                         13.682    Air    f = 13.29, NA 0.33,                                          11.219    Air    Finite lens                                 0     -0.0072603                                                                               1.5000000 PMMA                                               0      0.0443797                                                                               5.0000000 LAH78                                                               53.9022942                                                                              Air                                                ______________________________________                                        Decentering of chief ray incident on LMa cylinder lens                                                    -1.124                                            Tilt of chief ray incident on LMa cylinder lens                                                           -0.0177                                           Decentering of chief ray incident on LMb cylinder lens                                                    -1.099                                            Tilt of ray incident on LMb cylinder lens                                                                 -0.0174                                           Offset of optical axis of system of LM pre-deflection optical                                             0.294                                             system on reflection surface from optical axis of system of                   post-deflection optical system                                                Tile of optical axis of system of LM pre-deflection optical                                               0.016                                             system on reflection surface from opticai axis of system of                   post-deflection optical system                                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        For cyan                                                                      Curvature                                                                     CUY   CUX        Thickness Material                                                                             Remarks                                     ______________________________________                                                                   Air    f = 13.29, NA 0.33,                                          10.253    Air    Finite lens                                 --    -0.0072603                                                                               1.5000000 PMMA                                               --     0.0443797                                                                               5.0000000 LAH78                                              --               54.6292496                                                                              Air                                                ______________________________________                                        Decentering of chief ray incident on LCa cylinder lens                                                    1.571                                             Tilt of chief ray incident on LCa cylinder lens                                                           -0.0134                                           Decentering of chief ray incident on LCb cylinder lends                                                   1.509                                             Tilt of chief ray incident on LCb cylinder lens                                                           -0.0136                                           Offset of optical axis of system of LC pre-deflection                                                     1.793                                             optical system on reflection surface from optical axis                        of system of post-deflection optical system                                   Tilt of optical axis of system of LC pre-deflection                                                       0.014                                             optical system on reflection surface from optical axis                        of system of post-deflection optical system                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        For yellow, black                                                             Curvature                                                                     CUY   CUX        Thickness Material                                                                             Remarks                                     ______________________________________                                                         13.682    Air    f = 13.29, NA 0.33,                                          6.773     Air    Finite Lens                                 --    -0.0072603                                                                               1.5000000 PMMA                                               --     0.0443797                                                                               5.0000000 LAH78                                              --               57.4181824                                                                              Air                                                ______________________________________                                        Decentering of chief ray incident on LYa cylinder lens                                                   -2.831                                             Tilt of chief ray incident on LYa cylinder lens                                                          -1.695D-005                                        Decentering of chief ray incident on LYb cylinder lens                                                   -2.766                                             Tilt of chief ray incident on LYb cylinder lens                                                          1.661D-004                                         Offset of optical axis of system of LY pre-deflection                                                    -1.884                                             optical system on reflection surface from optical                             axis of system of post-deflection optical system                              Tilt of optical axis of system of LY pre-deflection                           optical systein on reflection surface from optical axis                                                  4.850D-044                                         of system of post-deflection optical system                                   Decentering of chief ray incident on LBa cylinder lens                                                   2.831                                              Tilt of chief ray incident on LBa cylinder lens                                                          1.695D-005                                         Decentering of chief ray incident on LBb cylinder lens                                                   2.766                                              Tilt of chief ray incident on LBb cylinder lens                                                          -1.661D-004                                        Decentering of chief ray incident on LBc cylinder lens                                                   2.701                                              Tilt of chief ray incident on LBc cylinder lens                                                          -3.153D-004                                        Decentering of chief ray incident on LBd cylinder lens                                                   2.636                                              Tilt of chief ray incident on LBd cylinder lens                                                          -4.644D-004                                        Offset of optical axis of system of LB pre-deflection                                                    1.884                                              optical system on reflection surface from optical axis                        of system of post-deflection optical system                                   Tilt of optical axis of system of LB pre-deflection                                                      -4.850D-004                                        optical system on reflection surface from optical                             axis of system of post-deflection optical system                              ______________________________________                                    

As can be seen from Tables 1 to 3, the finite lenses 9 and the cylinderlenses 11 alone corresponding to the respective color components useidentical lenses independently of color components. Note that thepre-deflection optical system 7Y corresponding to the Y (yellow) imageforming unit 58Y and the pre-deflection optical system 7B correspondingto the B (black) image forming unit 58B, which are respectively locatedon the most upstream and downstream sides in the rotation direction ofthe conveyor belt 52 serving as a transfer belt have substantially thesame lens layouts (which are symmetrical about the optical axis of thesystem). On the other hand, each of the pre-deflection optical system 7Mcorresponding to M (magenta) and the pre-deflection optical system 7Ccorresponding to C (cyan) has a larger interval between the finite focallens 9 and the cylinder lens 11 as compared to that in thepre-deflection optical systems 7Y and 7B.

On the other hand, a maximum distance is set between the cylinder lenses11 and the reflection surface of the light deflection unit 5 inassociation with the laser beams at the two ends, i.e., the laser beamsLY and LB. As can be seen from FIG. 2, this can eliminate limitations(mounting surface) associated with the layouts of the laser elements3(Y, M, C, and B)a and 3(Y, M, C, and B)b, and the third and fourthblack laser elements 3Bc and 3Bd, and the corresponding finite lenses9(Y, M, C, and B)a and 9(Y, M, C, and B)b, and finite lenses 9Bc and 9Bdupon constituting each of a plurality of light sources using a pluralityof lasers.

The functions of the pre-deflection optical systems having effects onthe laser beams propagating from the light sources toward the lightdeflection unit will be explained below with reference to FIG. 3.

Note that the lenses and half mirrors arranged between the light sourcesand the light deflection unit have substantially the same effects.Hence, the laser beam LYa propagating from the first yellow laserelement 3Ya toward the polygonal mirror main body 5a of the lightdeflection unit 5 will be described below as an example.

As shown in FIG. 3, the laser beam LYa emitted by the first yellow laserelement 3Ya is given predetermined convergence by the finite lens 9Ya,and then passes through the stop 10Ya, so that its beam sectional shapeis adjusted to a predetermined shape.

The laser beam LY (LYa+LYb), which is synthesized into a substantiallysingle laser beam by the half mirror 12Y to be separated by apredetermined distance in the sub-scanning direction from the laser beamLYb (not shown in FIG. 3) coming from the second yellow laser element3Yb (to be described later), passes through a non-reflection region ofthe laser synthesis mirror unit 13 (to be described later with referenceto FIGS. 8A and 8B), and is synthesized with the remaining three groupsof laser beams LM, LC, and LB when viewed from the sub-scanningdirection. Then, the synthesized laser beam is guided to the lightdeflection unit 5.

The cylinder lens 11Y is integrally formed by adhering the exit surfaceof a first cylinder lens 17Y and the incident surface of a secondcylinder lens 19Y or by pressing them from a predetermined directiontoward a positioning member (not shown). The surface, contacting air,i.e., the incident surface, of the lens 17Y is formed into a cylindricalsurface, and is given a substantially equal curvature in thesub-scanning direction. The first cylinder lens 17Y consists of plastic,e.g., PMMA (polymethyl methacrylate). THe second cylinder lens 19Yconsists of glass, e.g., TaSF21.

Note that the cylinder lenses 17Y and 19Y are fixed via a positioningmechanism (not shown) integrally formed with a holding member 15 to beseparated by an accurate distance from the finite lens 9Ya or 9Yb.

The laser beam LYa enters the cylinder lens 11Y to be decentered andtilted from the optical axis of the lens 11Y to cancel coma generatedwhen the laser beam LYa passes through the first and secondimage-forming lenses 30a and 30b (as indicated by an alternate long andshort dashed line extending from the incident surface of the cylinderlens indicated by 17Ya in FIG. 3).

The laser beam LYb (not shown) enters the cylinder lens 11Y to beasymmetrical with the laser beam LYa with respect to the optical axis ofthe lens 11Y. Note that the laser beam LYb enters the cylinder lens 11Yto substantially overlap the laser beam LYa if it is illustrated underthe same condition as in FIG. 3.

Tables 4 and 5A, 5B, 5C and 5D below summarize optical numerical valuedata of the post-deflection optical system used in combination with thepost-deflection optical systems shown in Tables 1 to 3 above.

                                      TABLE 4                                     __________________________________________________________________________            Post-deflection optical system                                                (Unit of angle: rad, unit of length: mm)                                      Direction where light rays travel are from + to                       __________________________________________________________________________            -                                                                     Curvature                                                                     CUY  CUX   Thickness                                                                            Material                                                                          Remarks                                                 __________________________________________________________________________               -36.7780266                                                                          Air Decentering in y-direction -4.364                       0.0189634                                                                          -0.0231677       First surface                                                      -6.5294938                                                                           PMMA                                                        0.0207366                                                                           0.0119239       Second surface                                                     -105.4906235                                                                         Air                                                         0.0034197                                                                          -0.0045480       Third surface                                                      -6.0077405                                                                           PMMA                                                        0.0021422                                                                          -0.0179737       Fourth surface                                                     -2.9985602                                                                           Air                                                         --   --               For tilts, see data listed below                                   -2.0000000                                                                           BK7 sign of tilt                                            --   --               For LC                                                                              -170.061                                                     -170.000                                                                             Air For LY, LB                                                                          -170.336                                          __________________________________________________________________________    Tilt of LM incident cover glass                                                                  0.267636366395237                                          Tilt of LC incident cover glass                                                                 -0.392835532361101                                          Tilt of LY incident cover glass                                                                  0.727422457105662                                          Tilt of LB incident cover glass                                                                 -0.727422457105662                                          __________________________________________________________________________

                                      TABLE 5A                                    __________________________________________________________________________    Polynomial coefficients for first surface                                         0     1     2     3     4     5                                           __________________________________________________________________________    0   0.00 -0.03  0.00  2.75E-5                                                                            -5.41E-9                                                                            -7.18E-9                                     1   0.00  6.30E-5                                                                            -4.56E-7                                                                             2.18E-8                                                                             1.03E-9                                                                            -1.03E-10                                    2   4.58E-4                                                                            -5.97E-6                                                                             6.10E-8                                                                             4.08E-8                                                                            -2.11E-9                                                                             1.67E-11                                    3  -6.17E-6                                                                             9.02E-7                                                                            -6.46E-8                                                                            -4.43E-9                                                                             2.38E-10                                                                            3.48E-12                                    4  -8.19E-9                                                                            -1.88E-8                                                                             1.22E-9                                                                             7.25E-11                                                                           -2.77E-12                                                                            0.00                                        __________________________________________________________________________    Polynomial coefficients for first surface                                          6      7      8      9     10                                            __________________________________________________________________________    0   -3.97E-10                                                                             3.53E-11                                                                            -7.41E-13                                                                            -1.16E-14                                                                            7.48E-16                                      1    7.45E-12                                                                            -2.59E-13                                                                            -3.18E-15                                                                             7.31E-18                                                                            5.10E-18                                      2    2.56E-12                                                                            -4.78E-14                                                                             5.85E-16                                                                            -2.45E-16                                                                            6.79E-18                                      3   -2.36E-13                                                                            -1.97E-15                                                                             4.73E-17                                                                            -1.23E-18                                                                            1.38E-19                                      4    0.00   0.00   0.00   0.00  0.00                                          __________________________________________________________________________

                                      TABLE 5B                                    __________________________________________________________________________    Polynomial coefficients for second surface                                        0     1     2     3     4     5                                           __________________________________________________________________________    0   0.00 -0.059                                                                               0.00  1.47E-5                                                                            -4.75E-6                                                                             9.18E-9                                     1   0.00  7.41E-5                                                                            -2.86E-6                                                                             7.25E-8                                                                            -4.40E-9                                                                            -4.55E-11                                    2   3.58E-4                                                                            -1.37E-6                                                                            -2.50E-7                                                                            -1.22E-8                                                                            -8.21E-11                                                                           -9.38E-11                                    3  -5.14E-5                                                                             2.42E-7                                                                             4.41E-8                                                                            -1.56E-9                                                                            -3.51E-11                                                                           -7.16E-13                                    4   1.49E-6                                                                            -2.29E-8                                                                             2.95E-10                                                                            1.06E-10                                                                           -4.17E-12                                                                            0.00                                        __________________________________________________________________________    Polynomial coefficients for second surface                                         6      7      8      9      10                                           __________________________________________________________________________    0   -1.09E-9                                                                              1.24E-11                                                                            -2.55E-13                                                                             1.49E-15                                                                             2.39E-16                                     1    1.31E-11                                                                            -4.38E-13                                                                            -2.87E-15                                                                            -3.30E-17                                                                             7.03E-18                                     2   -1.70E-12                                                                            -5.24E-14                                                                             2.05E-15                                                                             3.82E-17                                                                            -2.77E-18                                     3   -9.04E-14                                                                            -5.47E-15                                                                             1.75E-16                                                                            -1.16E-18                                                                             5.47E-21                                     4    0.00   0.00   0.00   0.00   0.00                                         __________________________________________________________________________

                                      TABLE 5C                                    __________________________________________________________________________    Polynomial coefficients for third surface                                         0     1     2     3     4     5                                           __________________________________________________________________________    0   0.00  0.016                                                                               0.00 -3.05E-6                                                                            -2.57E-7                                                                             4.42E-10                                    1   0.00 -2.50E-5                                                                             5.27E-7                                                                             1.36E-9                                                                             7.55E-11                                                                            4.20E-13                                    2  -6.25E-6                                                                            -5.97E-8                                                                            -2.98E-10                                                                            1.65E-11                                                                           -6.53E-14                                                                            3.01E-17                                    3  -1.22E-8                                                                            -1.26E-10                                                                           -2.71E-12                                                                            6.67E-14                                                                            6.18E-17                                                                           -6.16E-18                                    4   1.10E-10                                                                            5.84E-13                                                                            7.42E-14                                                                           -5.44E-16                                                                           -8.17E-18                                                                            0.00                                        __________________________________________________________________________    Polynomial coefficients for third surface                                          6      7      8     9       10                                           __________________________________________________________________________    0    1.42E-11                                                                             2.66E-14                                                                            -7.79E-16                                                                            1.77E-18                                                                             -1.71E-20                                     1   -1.84E-14                                                                             8.66E-17                                                                            -1.05E-18                                                                            3.80E-21                                                                              9.74E-24                                     2    1.21E-17                                                                            -5.55E-19                                                                             7.01E-22                                                                            3.37E-23                                                                             -1.54E-25                                     3    7.99E-20                                                                            -2.75E-22                                                                            -2.15E-25                                                                            9.22E-26                                                                              1.15E-27                                     4    0.00   0.00   0.00  0.00    0.00                                         __________________________________________________________________________

                                      TABLE 5D                                    __________________________________________________________________________    Polynomial coefficients for fourth surface                                        0     1     2     3     4     5                                           __________________________________________________________________________    0   0.00  0.011                                                                               0.00 -3.12E-6                                                                            -6.22E-8                                                                             4.30E-10                                    1   0.00 -1.81E-5                                                                             2.52E-7                                                                             6.63E-10                                                                            1.39E-11                                                                            4.05E-13                                    2  -1.01E-5                                                                            -6.82E-8                                                                             1.61E-10                                                                            1.98E-11                                                                           -3.41E-14                                                                           -1.20E-15                                    3  -2.15E-8                                                                             8.10E-11                                                                            3.66E-12                                                                            2.91E-14                                                                           -6.60E-16                                                                            4.83E-18                                    4   1.22E-10                                                                            1.52E-12                                                                           -7.17E-15                                                                           -6.57E-16                                                                            4.40E-18                                                                            0.00                                        __________________________________________________________________________    Polynomial coefficients for fourth surface                                         6      7      8      9      10                                           __________________________________________________________________________    0   -1.80E-13                                                                             6.10E-15                                                                             1.86E-16                                                                             4.47E-18                                                                            -6.12E-20                                     1   -1.85E-15                                                                             3.12E-17                                                                            -1.83E-18                                                                             1.35E-20                                                                            -5.53E-23                                     2   -1.06E-17                                                                            -1.04E-19                                                                             1.52E-21                                                                            -1.36E-24                                                                            -9.05E-26                                     3    2.27E-20                                                                            -7.17E-22                                                                            -3.48E-24                                                                             1.64E-26                                                                             2.08E-27                                     4    0.00   0.00   0.00   0.00   0.00                                         __________________________________________________________________________

By using the post-deflection optical system shown in Tables 4 and 5above, even when the maximum value of the surface inclination of eachreflection surface of the light deflection unit is assumed to be 1", theposition displacement of the beam on the image surface can be suppressedto 4 μm.

More specifically, the post-deflection optical system shown in Table 4has a 1/48× surface inclination correction function with respect to thesurface inclination of each reflection surface of the light deflectionunit 5. If the post-deflection optical system has no surface inclinationcorrection function, the inclination of each reflection surface of thelight deflection unit 5 must be reduced to about 2' so as to reduce thesurface inclination among the reflection surfaces of the lightdeflection unit 5 to a degree that jitter is not visually perceived. Asa consequence, the polygonal mirror main body 5a becomes very expensive.

The effective field angle, φ, of a beam deflected by the lightdeflection unit 5 is 1.01237 radians (to be abbreviated as radhereinafter), the effective image region width, W, including thehorizontal synchronization signal detection region is 320 millimeters(to be abbreviated as mm hereinafter), and the distance, LT, between thereflection point on each reflection surface of the light deflection unit5 and the image surface is 329.797 mm. By optimizing the beam effectivefield angle φ, the effective image region width W, and the distance LTbetween the reflection point and the image surface to satisfy:

    1.01237=φ>W/LT=0.97

the imaging characteristics, bend of scanning lines, and the likerequired for multi-beams can be avoided from deteriorating due toenvironmental changes, and the laser driving frequency for driving thelaser elements used in the light sources can be lowered. Hence, the sizeof the light scanning unit can also be reduced. Even when plastic lensesare used in the post-deflection optical system, an optical unitsuffering less color misregistration with respect to changes intemperature and humidity can be provided.

Note that φ>W/LT is obtained as a result of simulations using theeffective field angle φ and W/LT (the effective image region width/thedistance between the reflection point and the image surface) asvariables of optimization design. In the region φ<W/LT, the bend ofscanning lines and deterioration of the imaging characteristics becomelarge with respect to changes in temperature and humidity. Also, themanufacturing tolerance becomes stricter. On the other hand, in theregion φ>1.2(W/LT), the bend of scanning lines becomes large withrespect to changes in temperature and humidity.

In view of the foregoing, the beam effective field angle φ is preferablyselected from the range 1.2(W/LT)>φ>W/LT. Note that an evaluationfunction shown in FIG. 4 is used upon evaluating the beam effectivefield angle φ.

Referring to FIG. 4, the evaluation function becomes small as a wholenear φ1, but also becomes small near φ=1.1(W/LT).

FIG. 5 shows the laser beams LY, LM, and LC (LY has LYa and LYb, LMconsists of LMa and LMb, and LC has LCa and LCb) propagating fromreflection surfaces 13Y, 13M, and 13C in the laser synthesis mirror unitin a direction (i.e., the sub-scanning direction) perpendicular to therotation axis of each reflection surface of the light deflection unit 5toward the light deflection unit 5 in the pre-deflection optical systems7(Y, M, C, and B) shown in FIG. 3 and Tables 1 to 3.

As can be seen from FIG. 5, the laser beams LY, LM, LC, and LB areguided toward the light deflection unit 5 to be separated by differentintervals in a direction parallel to the rotation axis of the reflectionsurface of the light deflection unit 5. The laser beams LM and LC areguided toward each reflection surface of the light deflection unit 5 tobe asymmetrical with each other to sandwich, therebetween, a planeperpendicular to the rotation axis of the reflection surface of thelight deflection unit 5 and including the center, in the sub-scanningdirection of the reflection surface, i.e., a plane including the opticalaxis of the system of the light scanning unit 1. Note that the intervalsbetween two adjacent ones of the laser beams LY, LM, LC, and LB on eachreflection surface of the light deflection unit 5 are 2.26 mm (betweenLY and LM), 1.71 mm (between LM and LC), and 1.45 mm (between LC andLB).

FIG. 6 shows the state wherein the optical members arranged between thelight deflection unit 5 in the light scanning unit 1 to eachphotosensitive drum 58, i.e., the image surface are viewed at a positionwhere the deflection angle of the light deflection unit 5 is 0° and fromthe sub-scanning direction.

As shown in FIG. 6, first mirrors 3(Y, M, C, and B) for bending theoptical paths of a total of 10 (=2+2+2+4) laser beams L(Y, M, C, and B)that have passed through the image-forming lens 30b toward the imagesurface, and second and third mirrors 35Y, 35M, 35C, 37Y, 37M, and 37Cfor further bending the optical paths of the laser beams LY, LM, and LCbent by the first mirrors 33Y, 33M, and 33C are arranged between thesecond image-forming lens 30b of the post-deflection optical system 30and the image surface. As can be seen from FIG. 6, the laser beam LBcorresponding to a B (black) image is returned by the first mirror 33B,and is then guided toward the image surface without going through anyother mirrors.

The first and second image-forming lenses 30a and 30b, the first mirrors33(Y, M, C, and B), and second mirrors 35Y, 35M, and 35C are fixed by,e.g., an adhesive to a plurality of fixing members (not shown), whichare formed by integral molding on an intermediate base 1a of the lightscanning unit 1.

The third mirrors 37Y, 37M, and 37C are arranged to be movable in atleast one direction associated with a direction perpendicular to themirror surface via fixing ribs and a tilt adjustment mechanism (to bedescribed later with reference to FIG. 16).

Dust-proof glass plates 39(Y, M, C, and B) for protecting the interiorof the light scanning unit 1 from dust are arranged between the thirdmirrors 37Y, 37M, and 37C and the first mirror 33B, and the imagesurface, and at positions where the 10 (=2+2+2+4) laser beams L(Y, M, C,and B) reflected via the mirrors 33B, 37Y, 37M, and 37C exit the lightscanning unit 1.

The optical characteristics between the cylinder lenses and thepost-deflection optical system will be described in detail below.

As is known, since the post-deflection optical system 30, i.e., the two,first and second image-forming lenses 30a and 30b made of plastic, e.g.,PMMA, the refractive index n of these lenses changes from 1.4876 to1.4798 when the ambient temperature changes from 0°C. to 50° C. In thiscase, the imaging surface where the laser beam that has passed throughthe first and second image-forming lenses 30a and 30b is actuallyfocused, i.e., the imaging position in the sub-scanning direction,varies by about ±4 mm.

In contrast to this, by assembling lenses consisting of the samematerial as that used in the post-deflection optical system 30 in thepre-deflection optical systems 7 in FIG. 3 while optimizing theircurvatures, variations in imaging surface generated upon variations inrefractive index n caused by changes in temperature can be suppressed toabout ±0.5 mm. More specifically, as compared to a conventional opticalsystem in which the pre-deflection optical systems 7 are constituted byglass lenses and the post-deflection optical system 30 is constituted byplastic lenses, chromatic aberration in the sub-scanning directiongenerated due to changes in refractive index, caused by changes intemperature, of the lenses in the post-deflection optical system 30 canbe corrected.

However, the correction amount of chromatic aberration that can becorrected is proportional to the power of a plastic cylinder lens. Morespecifically, since the correctable amount of chromatic aberration isdetermined in accordance with the difference between the incidentsurface curvature and the exit surface curvature of the plastic cylinderlens, the curvature of a glass cylinder lens can be specified if theincident surface of the plastic cylinder lens is assumed to be a flatsurface. As a result, if the material used in the glass cylinder lens isspecified, the focal length of the cylinder lens can be determined.

Therefore, when the optical characteristics of the post-deflectionoptical system are determined, the minimum beam size in the sub-scanningdirection can be set by only the focal length of the cylinder lens. Inthis case, a sufficiently large degree of freedom in design cannot beassured, and a target beam size and achromaticity cannot be realized atthe same time.

As another method, the focal length of the cylinder lens may be set byadjusting the focal length of the glass cylinder lens by changing therefractive index upon changing a glass material. However, some glassmaterials are not always suitable for grinding, storage, ortransportation, and the degree of freedom inevitably lowers.

As still another method, curvatures may be given to both the incidentsurface and the exit surface of the glass cylinder lens, so that thepowers of the plastic cylinder lens and the glass cylinder lens aredefined by independent functions.

However, by the above-mentioned method of giving curvatures to the twosurfaces of the plastic cylinder lens formed by molding and defining thepowers of the plastic cylinder lens and the cylinder lens by independentfunctions, cost can be minimized. According to this method, highmachining precision and shape precision can be easily assured.

FIGS. 7A and 7B show the relationship between the first to fourthsynthesized laser beams L(Y, M, C, and B) that pass between the lightdeflection unit 5 shown in FIG. 6 and the image surface, and the opticalaxis of the system, in the sub-scanning direction, of the light scanningunit 1.

As shown in FIGS. 7A and 7B, the first to fourth synthesized laser beamsL(Y, M, C, and B) reflected by each reflection surface of the lightdeflection unit 5 cross the optical axis of the system in thesub-scanning direction between the first and second image-forming lenses30a and 30b, and are guided to the image surface.

FIGS. 8A and 8B show the laser synthesis mirror unit 13 which guides thefirst to fourth synthesized laser beams LY, LM, LC, and LB as a bundleof laser beams to each reflection surface of the light deflection unit5.

The laser synthesis mirror unit 13 is constituted by the first to thirdmirrors 13M, 13C, and 13B arranged in correspondence with the number ofcolor components (the number of color-separated colors)--"1", first tothird mirror holding members 13 α, 13 β, and 13 γ, and a base 13a forsupporting these holding members 13 α, 13 β, and 13 γ. Note that thebase 13a and the holding members 13 α, 13 β, and 13 γ are integrallyformed using a material with a small thermal expansion coefficient,e.g., an aluminum alloy.

At this time, the laser beam LY from the light source 3Y, i.e., from thefirst and second yellow laser elements 3Ya and 3Yb, is directly guidedto each reflection surface of the light deflection unit 5, as hasalready been described above. In this case, the laser beam LY passesthrough the base 13a side of the optical axis of the system of the lightscanning unit 1, i.e., between the mirror 13M fixed to the first holdingmember 13 α and the base 13a.

The light intensities (light amounts) of the laser beams LM, LC, and LB,which are reflected by the mirrors 13M, 13C, and 13B of the synthesismirror unit 13 shown in FIGS. 8A and 8B, and are then guided to thelight deflection unit 5, and the laser beam LY directly guided to thelight deflection unit 5 will be examined below.

With the laser synthesis mirror unit 13, the laser beams LM, LC, and LBare returned by the normal mirrors (13M, 13C, and 13B) in a region,where the laser beams LM, LC, and LB are separated in the sub-scanningdirection, before they become incident on each reflection surface of thelight deflection unit 5. Therefore, the light amounts of the laser beamsL(M, C, and B), which are reflected by the reflection surfaces (13M,13C, and 13B) and are then guided toward the polygonal mirror main body5a, can be maintained to be about 90% or more of those output from thecylinder lenses 11. Since not only the outputs from the laser elementscan be reduced but also no aberrations are generated by an inclinedplane-parallel plate, aberrations of light that reaches the imagesurface can be uniformly corrected. Hence, the beam size of each laserbeam can be reduced, and consequently, high-definition recording can berealized. Note that the laser beams emitted by the laser elements 3Yaand 3Yb corresponding to Y (yellow) are directly guided to eachreflection surface of the light deflection unit 5 without beingreflected by any mirrors in the synthesis mirror unit 13. Hence, notonly the output capacities of the lasers can be reduced, but also errorsof the incident angles owing to reflection by the mirrors (13M, 13C, and13B; such errors may be generated in other laser beams reflected by thesynthesis mirror unit) can be removed.

The relationship among the laser beams L(Y, M, C, and B) reflected byeach reflection surface of the light deflection unit, the tilts of thelaser beams LY, LM, LC, and LB, which are output from the light scanningunit 1 via the post-deflection optical system 30, and the mirrors 33B,33Y, 37M, and 37C will be explained below with reference to FIGS. 2 to6.

As has already been described above, the laser beams LY, LM, LC, and LB,which are reflected by the polygonal mirror main body 5a of the lightdeflection unit 5 and are given predetermined aberration characteristicsby the first and second image-forming lenses 30a and 30b, are returnedin a predetermined direction via the first mirrors 33Y, 33M, 33C, and33B.

At this time, the laser beam LB is reflected by the first mirror 33B,and is then guided to the photosensitive drum 58B via the dust-proofglass plate 39B. In contrast to this, the remaining laser beams LY, LM,and LC are guided by the second mirrors 35Y, 35M, and 35C, are reflectedby the second mirrors 35Y, 35M, and 35C toward the third mirrors 37Y,37M, and 37C, and are then reflected by the third mirrors 37Y, 37M, and37C. Thereafter, these laser beams are imaged on the correspondingphotosensitive drums to be separated by nearly equal intervals via thedust-proof glass plates 39Y, 39M, and 39C. In this case, the laser beamLB output via the first mirror 33B, and the laser beam LC adjacent tothe laser beam LB are also imaged on the photosensitive drums 58B and58C to be separated by nearly equal intervals.

After the laser beam LB is reflected by each reflection surface of thepolygonal mirror main body 5a, it is reflected by only the mirror 33B,and is then output from the light scanning unit 1 toward thephotosensitive drum 58B.

When a plurality of mirrors are present along the optical path, thelaser beam LB is effectively used as reference light rays uponrelatively correcting the remaining laser beams L(Y, M, and C) inassociation with variations in various aberration characteristics, bend,and the like, which increase (are multiplied) as the number of mirrorsincreases.

When a plurality of mirrors are present in the optical path, the numberof mirrors used for each of the laser beams LY, LM, LC, and LB ispreferably adjusted to be an odd or even value. More specifically, asshown in FIG. 5, the number of mirrors in the post-deflection opticalsystem associated with the laser beam LB is one (odd value) except forthe polygonal mirror main body 5a of the light deflection unit 5, andthe number of mirrors in the post-deflection optical system associatedwith each of the laser beams LC, LM, and LY is three (odd value) exceptfor the polygonal mirror main body 5a of the light deflection unit 5.Assuming that the second mirror 35 is omitted in association with one ofthe laser beams LC, LM, and LY, the direction of the bend, caused by thetilts of lenses and the like, of a laser beam propagating along theoptical path in which the second mirror 35 is omitted (the number ofmirrors is an even value) becomes opposite to that of the bend, causedby the tilts of lenses and the like, of other laser beams (their opticalpaths include even numbers of mirrors), thus causing colormisregistration that is a serious problem upon reproducing apredetermined color.

When a predetermined color is reproduced by superposing 10 (=2+2+2+4)laser beams LY, LM, LC, and LB, the numbers of mirrors inserted in theoptical paths of the laser beams LY, LM, LC, and LB in thepost-deflection optical system 30 are substantially standardized to beodd or even values.

FIGS. 9A and 9B show the relationship among the distance, L₁, betweenthe optical axis of the system of the post-deflection optical system anda scanning line on one end, the distance, L_(M), between the opticalaxis of the system of the post-deflection optical system and a scanningline on the other end, and the distance, ΔL_(MAX), in a directionparallel to the optical axis of the system, between the end scanninglines.

The maximum values of these values (L₁, L_(M), and ΔL_(MAX)) are set inaccordance with the distance, L₀, between the final lens surface and theimage surface, and the intervals between adjacent beams indicated by M=4groups. However, if the maximum values are set preferentially based onthe distance L₀ between the final lens surface and the image surface,requirements on the optical system become stricter, and variations invarious characteristics, e.g., imaging characteristics, bend of scanninglines, and the like due to environmental changes may reach levels thatcannot be ignored.

In view of this problem, as a result of repetition of designing manylight scanning units and mounting optical members, conditions that canmaintain high optical performance of the light scanning unit and canassure required intervals between adjacent photosensitive drums andrequired distances between the light scanning unit and thephotosensitive drums were confirmed. That is, high optical performanceof the light scanning unit can be maintained and required intervalsbetween adjacent photosensitive drums and required distances between thelight scanning unit and the photosensitive drums can be assured underthe conditions that the following relations hold among theabove-mentioned distances L₁, L_(M), and ΔL_(MAX) :

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/1.8>L.sub.0

    L.sub.0 >(ΔL.sub.MAX +L.sub.M +L.sub.1)/2

More specifically, a laser beam passing through the final lens (thesecond image-forming lens 30b) can be approximated to the one that isoutput in a direction parallel to a line connecting the optical axes ofthe two lenses 30a and 30b of the post-deflection optical system 30, asshown in FIGS. 10A and 10B.

On the other hand, reflections of a beam separated at a position closestto the lens side by the first and second mirrors are approximated by asingle reflection, and let (x, y)=(0, 0) be this reflection point. Inaddition, the photosensitive drum is assumed to be located on the sideabove the optical axis (i.e., above the plane of the drawing of FIGS.10A and 10B) (hence, the coordinate system used here is different fromthose of other drawings).

Furthermore, let (x₁, y₁) be the reflection point by the third mirror,(x₂, y₂) be the coordinate position of the image surface, and (x₃, y₃)be the coordinate position of the image surface of the beam which passesthrough only one mirror.

Then, in order to maximize the mountable volume of process-relatedmembers, if y₁ is positive, an area S₁ bounded by (x₃, y₃), (x ₂, y₂),(x₁, y₁), and (x₃, y₁) need only be maximized, as shown in FIG. 10A. Onthe other hand, if y₁ is negative, an area S₂ bounded by (x₃, y₃), (x₂,y₂), (x₁, 0), and (x₃, 0) need only be maximized.

Let ψ be the angle the beam reflected by the second mirror and theoptical axis of the post-deflection optical system make, and L₁ be thedistance between the reflection points by the second and third mirrors.Then, in order to maximize the area S₁ or S₂, y₁ and x₁ are respectivelydescribed by:

    y.sub.1 =L.sub.1 *Sin[ψ]                               (a-1)

    x.sub.1 =L.sub.1 *Cos[ψ]                               (a-2)

where * is the complex conjugate number.

Let L₂ be the optical path length from the reflection point by thesecond mirror to the image surface, and (x₂, y₂) be the coordinateposition of the image surface by approximating that the beam reflectedby the third mirror propagates in a direction perpendicular to theoptical axis of the post-deflection optical system (in such case, S₁ orS₂ is maximized). Then, we have:

    x.sub.2 =x.sub.1                                           (a- 3)

    y.sub.2 =y.sub.1 +L.sub.2 -L.sub.1                         (a- 4)

On the other hand, let y₄ be the distance between a beam separated at aposition closest to the lens side and a beam separated finally by thefirst mirror. Then, the following relations hold:

    x.sub.3 =-(L.sub.2 -y.sub.4 -y.sub.2)                      (a-5)

    y.sub.3 =y.sub.2                                           (a-6)

Therefore, S₁ and S₂ are respectively given by:

    S.sub.1 =(y.sub.2 -y.sub.1) (x.sub.2 -x.sub.3)             (a-7)

    S.sub.2 =y.sub.2 (x.sub.2 -x.sub.3)                        (a-8)

Since y₄ is set to be a distance that allows three beams to separate atpositions where the beams do not overlap each other, when the separationpoints are located at equal intervals with respect to the image surfacesof the respective beams, the optical path lengths from the imagesurfaces and the corresponding separation points are respectively givenby L₂, (L₂ -(x₂ -x₃)/3/2), and 2(L₂ -(x₂ -x₃)/3/2).

On the other hand, if ω₀ represents the beam radius on the imagesurface, the convergence angle of the beam is expressed by λ/(πω).Hence, if ξ represents a coefficient including the influence ofdiffraction by the stops of the pre-deflection optical system, y₄ isdefined by:

    y.sub.4 =ξ2λ/π/ω.sub.0 (L.sub.2 +(L.sub.2 -(x.sub.2 -x.sub.3)/3/2)+2(L.sub.2 -(x.sub.2 -x.sub.3)/3/2))        (a-9)

Note that ξ is normally set to be 1.4 to obtain the beam size ω₀ on theimage surface. On the other hand, ξ is defined to be about 2.8 to removethe influence of diffraction of neighboring beams in association with,especially, the first mirror (separation mirror).

Solving formula (a-9) for y₄ yields:

    y.sub.4 =(ξL.sub.1 λ6ξL.sub.2 λ+ξL.sub.1 λCos[ψ]-ξL.sub.1 λSin[ψ])/(-(ξλ)+πω0))       (a-10)

Note that data shown in FIG. 11 is obtained by substituting formula(a-10) and formulas (a-1) to (a-6) in formulas (a-7) and (a-8), andplotting the smaller one of the values S₁ and S₂ on the ordinate. InFIG. 11, under the conditions of ξ=1.4, L₂ =175, λ=0.00068, and ω₀=0.025, the abscissa plots ψ within the range from -π to π, and the axisin the direction of depth plots L₁ within the range from 0 to 175.

As can be seen from FIG. 11, a condition that the smaller one of thevalues S₁ and S₂ is maximized is:

    ψ=0                                                    (a-11)

Calculating a solution which yields 0 by differentiating S₁ by L₁, wehave: ##EQU2##

In practice, the numerical value obtained by adding the distance betweenthe final lens surface and the first mirror to L₂ is L₀. However,assuming L₂ =L₀ by approximation, we can rewrite the solution as:

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/L.sub.0 ≈((2y2+x.sub.1 -x.sub.3)/L.sub.2                                         (a- 13)

Therefore, substitution of formula (a-11) into formula (a-13) yields:

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/L.sub.0 ≈((ξλ-2πω.sub.0) (3ξλ-πω.sub.0))/(πω.sub.0 (-(ξλ)+πω.sub.0))                      (a-14)

FIG. 12 shows the value of formula (a-14) while the abscissa plots ξwithin the range from 1.4142 to 2.8 and the axis in the depth directionplots ω₀ within the range from 0.02 to 0.06 with respect to λ=0.00063 asa currently practical combination of the ranges. Similarly, FIG. 13shows the value of formula (a-14) while the abscissa plots ξ within therange from 1.4142 to 2.8 and the axis in the depth direction plots ω₀within the range from 0.02 to 0.06 with respect to λ=0.0008.

It is seen from FIGS. 12 and 13 that high optical performance of thelight scanning unit can be maintained and prescribed intervals betweenadjacent photosensitive drums and prescribed distances between the lightscanning unit and the photosensitive drums can be assured, i.e., themountable volume of process-related members is maximized with respect toS₁, under the conditions that the following relations hold:

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/1.8>L.sub.0

    L.sub.0 >(ΔL.sub.MAX +L.sub.M +L.sub.1)/2

In the above description, the intervals between adjacent photosensitivedrums are assumed to be constant. For example, when the diameter of thephotosensitive drum to which a beam to be returned only once is guidedis larger than that of other photosensitive drums by ΔD, the transferpositions can be maintained at predetermined heights by decreasing L_(M)by ΔD and increasing ΔL_(MAX) by ΔD.

For this reason, the value (ΔL_(MAX) +L_(M) +L₁) remains the same evenwhen the diameters of the photosensitive drums are not uniform.Therefore, formula (a-15) is also effective for an image formingapparatus in which one of the intervals between adjacent photosensitivedrums and the diameters of the photosensitive drums are nonuniformlyset.

In order to verify these conditions, when the above-mentioned conditionsare applied to the light scanning unit shown in FIGS. 2 to 9B, and FIGS.14A to 16, since:

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/1.8=187.25 mm

    L.sub.0 =175 mm

    (ΔL.sub.MAX +L.sub.M +L.sub.1)/2=168.527

it is apparent that the above-mentioned conditions are satisfied.

Therefore, the size of the entire post-deflection optical system and thelenses used therein can be appropriately selected, and the differencesin curvature generation amount of the M beam groups due to environmentalchanges can be suppressed. At the same time, the size of the lightscanning unit can be prevented from becoming undesirably large. Sincethe field angle can be increased as compared to a light scanning unithaving nearly an equal size, a lower image frequency can be set.

FIGS. 14A and 14B are schematic views showing the positionalrelationship of the laser beams guided to the image surface.

FIG. 14A shows the laser beams LM and LC each of which is obtained bysynthesizing two laser beams, and FIG. 14B shows an example of the laserbeam LB obtained by synthesizing four laser beams. In FIGS. 14A and 14B,the hatched portion corresponds to a region where the light intensity ofthe laser beam becomes equal to or higher than 1/e². Note that thesectional shape of the beam is set so that the 1/e² diameter in the mainscanning direction becomes about 0.8 to 1.2 times the distance (pitch)between adjacent beams, and the 1/e² diameter in the sub-scanningdirection becomes about 1.2 to 1.6 times the pitch. The adjacent beams(four beams for LB) are set to scan adjacent scanning lines on the imagesurface and are offset in the scanning direction by the 1/e² diameter ormore to prevent the 1/e² diameters from overlapping each other.

More specifically, when an image is exposed using a laser beam obtainedby synthesizing two or more laser beams, if the 1/e² diameters of thelaser beams in which the light intensities of the respective laser beamsbecome 1/e² or higher overlap each other, the beam shape may change dueto interference between the beams. However, as shown in FIGS. 14A and14B, by slightly shifting the beam intervals and the positions in thescanning direction, changes in beam shape due to interference betweenthe beams can be prevented.

FIG. 15 shows in detail the horizontal synchronization mirror.

Referring to FIG. 15, the horizontal synchronization mirror 25 has firstto fourth mirror surfaces 25Y, 25M, 25C, and 25B which are formed tohave different angles in both the main scanning and sub-scanningdirections and a mirror block 25a for integrally holding these mirrorsurfaces 25(Y, M, C, and B), so as to reflect the synthesized laserbeams LY, LM, LC, and LB toward the horizontal synchronization detector23 at different timings in the main scanning direction, and to providesubstantially the same heights on the horizontal synchronizationdetector 23 in the sub-scanning direction.

The mirror block 25a is formed using, e.g., glass-containing PC(polycarbonate). On the other hand, the mirrors 25(Y, M, C, and B) areformed by depositing a metal such as aluminum at corresponding positionsof the block 25a formed at predetermined angles.

With this arrangement, the laser beams LY, LM, LC, and LB deflected bythe light deflection unit 5 can be incident at identical detectionpositions on the single detector 23. Also, for example, when a pluralityof detectors are arranged, horizontal synchronization signals can beprevented from being shifted due to variations in sensitivity or theposition deviations of the detectors. Note that the horizontalsynchronization detector 23 receives the laser beam groups LY, LM, LC,and LB a total of four times per line in the main scanning direction,and can obtain horizontal synchronization signals N_(i) times (twice foreach of LY, LM, and LC, and four times for LB) for each beam, needlessto say. Also, the mirror block 25a can be separated from the moldwithout requiring any undercut since the mirror surfaces of the mold areintegrally designed to be formed by grinding a block.

FIG. 16 is a schematic perspective view showing the support mechanism ofthe third mirrors 37Y, 37M, and 37C.

Referring to FIG. 16, the third mirrors 37(Y, M, and C) are held atpredetermined positions of the intermediate base 1a of the lightscanning unit 1 by fixing portions 41(Y, M, and C) formed integrallywith the intermediate base 1a and mirror press leaf springs 43(Y, M, andC) which respectively oppose the fixing portions 41(Y, M, and C) tosandwich the corresponding mirrors therebetween.

A pair of fixing portions 41(Y, M, or C) are formed on two end portionsof each mirror 37(Y, M, or C).

Two projections 45(Y, M, or C) for holding the mirror 37(Y, M, or C) attwo points are formed on one fixing portion 41(Y, M, or C). A set screw47 for movably supporting the mirror held by the projections 45(Y, M, orC) in the vertical direction or along the optical axis is arranged onthe other fixing portion 41(Y, M, or C).

As shown in FIG. 16, since the mirrors 37(Y, M, and C) move in thedirection perpendicular to the mirror surface or in the optical axisdirection to have the projections 45(Y, M, and C) as fulcrums when theirset screws 47(Y, M, and C) move in a predetermined direction, the tiltin the main scanning direction, i.e., the bend is corrected.

FIGS. 17A to 17C are schematic views showing the exposure state of thelaser beams on the image surface, i.e., the state wherein a latent imageis formed on the photosensitive drum. Note that a description withreference to FIGS. 17A to 17C will be made while taking as an exampleblack laser beams, i.e., the four laser beams emitted by the lightsource 3B.

Referring to FIGS. 17A to 17C, regions indicated by solid linescorrespond to those subjected to image generation by a certainreflection surface of the polygonal mirror main body 5a of the lightdeflection unit 5 (i.e., regions that deflect laser beams for imageformation), and the central region corresponds to the effective imageregion. Other regions correspond to non-effective regions for deflectinglaser beams which do not contribute to any image generation. Note thatportions indicated by alternate long and short dashed lines correspondto regions subjected to image generation by the next reflection surfaceof the polygonal mirror main body 5a.

FIG. 17A shows the angle the scanning line and the rotation direction ofthe photosensitive drum make, i.e., the tilt amount, k, of the scanningline with respect to the sub-scanning direction, if p represents theinter-beam distance (beam pitch) and k represents the number ofreflection surfaces of the polygonal mirror main body 5a, when (N-4)laser beams deflected by a certain reflection surface of the polygonalmirror main body 5a of the light deflection unit 5 are imaged to scan apredetermined image surface at an equal speed.

Note that the tilt δ is obtained by:

    δ=tan.sup.-1 [(N×p×k×φ)/(4×π×W)]

where N is the number of laser beams, φ is the effective field angle,and W is the effective image region width. The tilt δ lags (-) withrespect to the rotation direction of the photosensitive drum.

FIG. 17B shows an example of correcting the tilt 3 obtained by FIG. 17A,i.e., illustrates that the scanning line becomes parallel to therotation direction of the photosensitive drum (i.e., the scanning linebecomes parallel to the axis of the photosensitive drum) by setting acertain tilt δ between the scanning line and the rotation direction ofthe photosensitive drum, i.e., the sub-scanning direction. In this case,the tilt δ matches the angle shown in FIG. 17A. Note that the tilt δleads (+) with respect to the rotation direction of the photosensitivedrum, needless to say. Also, the tilt δ is given to the entire lightscanning unit or at least all the optical members participating inguiding laser beams reflected by the light deflection unit toward thephotosensitive drums (when the light scanning unit 1 is arranged in theimage forming apparatus 100, the axes connecting the light deflectionunit 5 and the photosensitive drums 58(Y, M, C, and B) tilt by a δ withrespect to those of the photosensitive drums 58). In order to set anoptimal tilt angle δ_(i) with respect to the respective beam groups, therespective beam groups can be tilted by δ_(i) -δ with respect to themirrors 35 and 37 in the main scanning direction.

FIG. 17C shows the write start positions by the four beams of thescanning lines corrected to become parallel to the rotation direction ofthe photosensitive drum by the method shown in FIG. 17B. As shown inFIG. 17C, the write start positions can be improved to be perpendicularto the rotation direction of the photosensitive drum by tilting thehorizontal synchronization reference positions of the four laser beamthrough the angle δ with respect to the scanning lines.

Let v be the process speed (mm/s) and Np be the rotational speed (rpm)of the rotary polygonal mirror main body 5a of the light deflection unit5. Then, the moving distance of the surface of the photosensitive drumduring the exposure time of the laser beam by one reflection surface ofthe rotary polygonal mirror main body 5a is 60 v/(NpN)=Q (mm). Morespecifically, Q is a function of the process speed, and the tilt δ ofthe scanning line is described as follows: ##EQU3## and is a function ofthe process speed.

As described above, when each of M groups of laser beams includes Nbeams, the entire light scanning unit or all the optical members takingpart in guiding the laser beam reflected by the light deflection unittoward the photosensitive drums is or are tilted in the directionopposite to the tilt generated upon guiding N_(i) beams toward thephotosensitive drums, thereby correcting the tilt of the scanning line.With this correction, even when the number N of beams per group isincreased, the horizontal line of the output image can be prevented fromtilting.

The phase difference among N_(i) laser beams will be explained belowwith reference to FIGS. 18A and 18B to FIG. 22. In the followingdescription, a case of two beams will be exemplified, but the sameapplies to three or more beams.

FIG. 18A shows the intensity distribution when the phase differencebetween neighboring beams is 0°, and FIG. 18B shows the intensitydistribution when the phase difference is 180°. In the followingdescription, let βp be the e⁻² diameter in the main scanning directionof the beam (p is the beam pitch), αp be the e⁻² diameter in thesub-scanning direction of the beam, ζ be the half exposure amount ofeach photosensitive drum/average exposure energy, and η be the intensityof one beam relative to the peak intensity of the other beam at themiddle point of a line connecting the centers of two neighboring beams.Also, in the following description, α=1.2, β=0.8, ζ=0.25, η=0.211, andp=0.042 mm. Note that the above-mentioned conditions indicate that thebeam central position is shifted by 0.042 mm in the sub-scanningdirection and by 0.0096 mm in the main scanning direction.

As can be seen from FIGS. 18A and 18B, when N_(i) laser elements of alight source having N_(i) beams per group simultaneously emit N_(i)laser beams, the intensity distribution of the laser beams that reachthe photosensitive drums changes as a result of the influence ofinterference due to the phase difference. More specifically, as shown inFIG. 18A, if the phase difference is 0°, the intensity distributionbetween the two beams increases to enhance the exposure amount; if thephase difference is 180° (FIG. 18B), the valley of the intensitydistribution is formed between the two beams.

FIGS. 19A and 19B are graphs obtained by normalizing the intensitydistributions on the photosensitive drum shown in FIGS. 18A and 18B bythe average exposure energy, i.e., obtained by dividing the integrationresult, in the y-direction, of the intensity distribution for only onebeam by p. Note that FIG. 19A corresponds to the phase difference=0° andFIG. 19B corresponds to the phase difference=180° as in FIGS. 18A and18B.

FIG. 19A reveals that a light intensity larger than 0.25 indicating thehalf decay amount which shows an amount of energy necessary fordecreasing by 50% the charge applied to the photosensitive drum 58 bycharger 60 (in this graph, since the average exposure energy is unity,half decay amount=half decay amount/average exposure energy×averageexposure energy=ζ×average exposure energy) can be assured.

In contrast to this, FIG. 19B indicates that a light intensity thatsatisfies the half exposure amount cannot be obtained in the valley ofthe intensity distribution.

This means that even a non-exposed portion is developed as a latentimage in the valley of the intensity distribution formed when the phasedifference between the two beams is180° in the case of normaldevelopment contrary to the central portions of the respective beams. Onthe other hand, in the case of reversal development, even an exposedportion is not developed, i.e., image omission occurs. Note that thephase difference between the two beams varies as time passes if thewavelengths of the laser beams emitted by the laser elements are notquite perfectly the same.

FIGS. 20A and 20B show the calculation results of the intensitydistributions as in FIGS. 18A and 18B for a certain group of laser beams(two beams) of the light scanning unit 1 shown in FIGS. 2 to 12according to the embodiment of the present invention. Assume that α, β,ζ, η, and p are respectively set to be α=1.2, β=0.8, ζ=0.25, η=0.135,and p=0.042 mm. Note that the above-mentioned conditions indicate thatthe beam central position is shifted by 0.042 mm in the sub-scanningdirection and by 0.0187 mm in the main scanning direction. The phasedifferences in FIGS. 20A and 20B are respectively 0° and 180°.

As is apparent from FIGS. 20A and 20B, if the phase difference is 0°(FIG. 20A), the intensity distribution between the two beams increasesto enhance the exposure amount; if the phase difference is 180° (FIG.20B), the valley of the intensity distribution is formed between the twobeams.

FIGS. 21A and 21B show the normalization results of the intensitydistributions shown in FIGS. 20A and 20B by the average exposure energyas in FIGS. 18A and 18B. Note that FIG. 21A corresponds to the phasedifference=0° and FIG. 21B corresponds to the phase difference=180° asin FIGS. 18A and 18B.

As can be seen from FIGS. 21A and 21B, in the light scanning unit shownin FIGS. 2 to 9B according to the embodiment of the present invention, alight intensity larger than 0.25 indicating the normalized half exposureamount can be assured independently of the phase difference (0° or 180°)between the neighboring beams.

A condition that can provide a light intensity larger than the halfexposure amount of the photosensitive drum even when the phasedifference between the two beams is 180° will be explained below.

As has already been described above, when p represents the beam intervalin the sub-scanning direction, βp represents the e⁻² diameter, in themain scanning direction, of the beam, αp represents the e⁻² diameter, inthe sub-scanning direction, of the beam, ζ represents the half exposureamount of the photosensitive body/average exposure energy, and ηrepresents the intensity of one beam relative to the peak intensity ofthe other beam at the middle point of a line connecting the centers oftwo neighboring beams, it is assumed that z represents the sub-scanningdirection, y represents the main scanning direction, one beam is locatedat a coordinate position (y, z)=(0, 0), and the other beam is located ata coordinate position (y, z)=(δy, p).

Then, the relative intensity η at the middle point of the lineconnecting the centers of the two neighboring beams is given by:

    η=exp.sup.-χ                                       (c- 1)

At this time, χ is described by: ##EQU4##

In contrast to this, δy that gives the relative intensity η at themiddle point of the line connecting the centers of the two neighboringbeams is calculated by solving formulas (c-1) and (c-2) for δy: ##EQU5##for χ=0.51n (η)

Therefore, the electric field distributions of the beams, each of whichhas the e⁻² diameter of βp in the main scanning direction and the e⁻²diameter of αp in the sub-scanning direction, and which have peakintensities at the coordinate positions (y, z)=(0, 0) and (δy, p) arerespectively defined by: ##EQU6##

Note that r_(z) and r_(y) in formulas (c-4) and (c-5) are respectivelygiven by:

    r.sub.z =αp/2                                        (c-6)

    r.sub.y =βp/2                                         (c-7)

Subsequently, the average intensity required for normalizing theintensity distribution is calculated.

When formula (c-5) is integrated in the main scanning direction, i.e.,the y-direction, we have: ##EQU7## where * is the complex conjugatenumber.

Thereafter, formula (c-8) is integrated in the sub-scanning direction,i.e., in the z-direction, and the integration result is divided by beampitch p to obtain the average intensity: ##EQU8##

When both the first and second lasers are turned on, the energy receivedat a certain location is proportional to a value obtained by integratingthe intensity in the main scanning direction, i.e., the y-directionsince the beams are scanned in the main scanning direction.

At this time, if energy larger than the half exposure amount of thephotosensitive drum is applied at a specific location corresponding tothe smallest energy, image omission or toner supply to a non-exposedportion can be prevented even when the phase difference between thelaser beams from the two lasers is 180°. Since the position of thelocation corresponding to the smallest energy is a coordinate position,in the z-direction, of the middle point of the line connecting thecenters of two neighboring beams, it can be calculated by:

    z=p/2                                                      (c-10)

The electric field at the location calculated by formula (c-10) is givenby e₁ -c2. Note that the value obtained by normalizing the integral ofthe light intensity in the main scanning direction, i.e., they-direction by the average exposure energy is given by: ##EQU9## Informula (c-11), since e₁ -e₂ is a real number from formulas (c-4) and(c-5), it can be approximated by (e_(1-e) ₂)*×(e₁ -e₂)=(e₁ -e₂)2).

The numerical value given by formula (c-11) becomes larger than the halfexposure amount of the photosensitive drum/average exposure energy, asdescribed by:

    i.sub.2 ≧ζ                                     (c-12)

Substitution of formula (c-11) into formula (c-12) obtained in thismanner yields: ##EQU10##

Therefore, by setting η to satisfy formula (c-13), the phase differencebetween neighboring laser beams need not be taken into considerationupon using a group of laser beams obtained by synthesizing two or morelaser beams.

In the light scanning device shown in FIGS. 2 to 12 according to theembodiment of the present invention, η<0.155354 is derived from formula(c-13).

In the examples shown in FIGS. 20A, 20B, 21A, and 21B, η=0.135, while inthe examples shown in FIGS. 18A, 18B, 19A, and 19B, η=0.211. Therefore,it is confirmed that formula (c-13) is effective.

FIG. 22 is a graph showing the value range that η as a limit value canassume with respect to some normally used values of α and ζ. Morespecifically, when the respective elements of the light scanning unitare set so that η falls with the range below the hatched region in FIG.22, the phase difference between neighboring laser beams need no longerbe considered.

The operation of the image forming apparatus 100 will be described belowwith reference to FIGS. 1 to 23.

When an image formation start signal is supplied from an operation panelor host computer (not shown), the image forming units 50(Y, M, C, and B)start a warm-up operation under the control of a main control unit 101,and the polygonal mirror main body 5a of the light deflection unit 5 inthe light scanning unit 1 is rotated at a predetermined rotational speedunder the control of an image control CPU 111.

Subsequently, a RAM 102 receives image data to be printed supplied froman external storage device, the host computer, or a scanner (imagereading device) under the control of the main control unit 101. Some orall data of the image data stored in the RAM 102 are stored in imagememories 114(Y, M, C, and B) under the control of the image control CPU111 in an image control unit 110.

Under the control of the main control unit 101, the pick-up roller 72 isbiased at a predetermined timing, e.g., with reference to a verticalsynchronization signal from a timing controller 113, to pick up onepaper sheet P from the paper cassette 70. The timing of the picked-uppaper sheet P is adjusted by the registration rollers 74 to Y, M, C, andB toner images provided by the image forming operations of the imageforming units 50(Y, M, C, and B), and the paper sheet P is chucked onthe conveyor belt 52 by the chucking roller 76. Then, the paper sheet Pis guided toward the image forming units 50 upon rotation of theconveyor belt 52.

Parallel to or simultaneously with the paper feed and convey operationsof the paper sheet P, laser driving units 116(Y, M, C, and B) are biasedon the basis of a clock signal CLK output from a timing setting unit(clock circuit) 118, and image data DAT held in the RAM 102 is suppliedto the light sources 3(Y, M, C, and B) under the control of datacontrollers 115(Y, M, C, and B). With this control, laser beams for oneline are irradiated onto the corresponding photosensitive drums 58(Y, M,C, and B) in the image forming units 50(Y, M, C, and B) in turn from apredetermined position of the effective print width in the main scanningdirection.

In order to change the intensities of the laser beams L(Y, M, C, and B)emitted by the light sources 3, image data is transferred to the laserdriving units 116(Y, M, C, and B) under the control of the datacontrollers 115(Y, M, C, and B), thus forming images free from anydeviation on the photosensitive drums 58(Y, M, C, and B) of the imageforming units 50(Y, M, C, and B) in one scan of the laser beams.

The first to fourth laser beams L(Y, M, C, and B) imaged on thecorresponding photosensitive drums 58(Y, M, C, and B) of the first tofourth image forming units 50(Y, M, C, and B) form electrostatic latentimages corresponding to the image data on the correspondingphotosensitive drums 58(Y, M, C, and B) by changing the potentials ofthe photosensitive drums 58(Y, M, C, and B), each of which is charged toa predetermined potential, on the basis of the image data.

The electrostatic latent images are developed by toners havingcorresponding colors by the developing units 62(Y, M, C, and B) and areconverted into toner images.

The toner images are moved toward the paper sheet P conveyed by theconveyor belt 52 upon rotation of the corresponding photosensitive drums58(Y, M, C, and B), and are transferred at predetermined timings ontothe paper sheet P on the conveyor belt 52 by the transfer units 64.

In this manner, four color toner images which accurately overlap eachother are formed on the paper sheet P. After the toner images aretransferred to the paper sheet P, the residual toners on thephotosensitive drums 58(Y, M, C, and B) are removed by the cleaners66(Y, M, C, and B), and the residual potentials on the photosensitivedrums 58(Y, M, C, and B) are removed by the charge removing lamps 68(Y,M, C, and B). Then, the photosensitive drums are used in the subsequentimage formation.

The paper sheet P that electrostatically holds the four color tonerimages is conveyed upon rotation of the conveyor belt 52, and isseparated from the conveyor belt 52 by the curvature of the belt drivingroller 56 and the straight traveling property of the paper sheet P.Then, the paper sheet P is guided to the fixing unit 84. On the papersheet P guided to the fixing unit 84, the respective color toners aremelted by the fixing unit 84 to fix the toner images as a color image.Thereafter, the paper sheet P is exhausted onto an exhaust tray (notshown).

On the other hand, after the paper sheet P is supplied to the fixingunit 84, the conveyor belt 52 is further rotated and unwanted remainingtoner on the surface is removed by the belt cleaner 82. Thereafter, thebelt 52 is used for conveying the next paper sheet P fed from thecassette 70.

As described above, according to the light scanning unit of the presentinvention, M beam groups are incident on the reflection surface of adeflection means so that the interval between adjacent beam groupsmonotonously increases from one end, and a beam group on one end havingthe smallest interval between adjacent beam groups is incident to crossbeams deflected by the deflection unit. With this arrangement,variations in imaging characteristics among beam groups and bend ofscanning lines between adjacent beam groups can be eliminated.Therefore, image quality can be prevented from deteriorating. Note thatthe size of the light scanning unit can also be reduced.

According to the light scanning unit of the present invention, let L₁ bethe distance between the optical axis of the system of a second opticalmeans and a scanning line on one end, L_(M) be the distance between theoptical axis of the system of the second optical means and a scanningline on the other end, and ΔL_(MAX) be the distance, in a directionparallel to the optical axis of the system, between the end scanninglines. Then, the distance L₀ between the final lens surface and theimage surface is set to fall within the range (ΔL_(MAX) +L_(M)+L₁)/1.8>L₀ >(ΔL_(MAX) L_(M) L₁)/2. In this manner, the bend of thescanning lines between adjacent beam groups can be eliminated. Note thatthe sizes of the lenses and light scanning unit can also be preventedfrom undesirably increasing.

Furthermore, according to the light scanning unit of the presentinvention, let L_(t) be the distance between the reflection point on thedeflection means and the image surface, and W be the effective imageregion width including a region where a horizontal synchronizationsignal is detected. Then, the effective field angle φ of a beam to bedeflected by the deflection means is set within the range φ>W/L_(t). Inthis manner, deterioration of the imaging characteristics and anincrease in degree of bend of scanning lines due to environmentalchanges can be prevented. Therefore, each lens can use a low-costplastic lens, and the cost of the light scanning unit can be reduced.

Moreover, according to the light scanning unit of the present invention,since the incident angles and positions to M sets of optical memberswith positive power in only the sub-scanning direction areasymmetrically set with respect to the optical axes of the opticalmembers, the influence of coma generated in laser beams that passthrough positions separated by a certain distance from the optical axisof the system in the sub-scanning direction can be eliminated. With thisarrangement, a decrease in resolution of images can be prevented.

According to the light scanning unit of the present invention, since thenumber of times that the beams pass through semi-transparent mirrors is1 or 0, variations in various aberrations among beam groups generatedwhen convergent laser beams are incident on a plane-parallel plate arehard to occur, and deterioration of image quality can be prevented.

Furthermore, according to the light scanning unit of the presentinvention, let p be the scanning pitch in the sub-scanning direction,and k be the number of rotary polygonal mirror surfaces. Then, since thescanning line is tilted by:

    δ=tan.sup.-1 (N.sub.i ×p×k×φ/(4×π×W))

from a direction perpendicular to the traveling direction of an imagecarrier, the scanning line can be prevented from tilting with respect tothe axis of the photosensitive drum, i.e., the sub-scanning direction.

Moreover, according to the light scanning unit of the present invention,let p be the beam pitch in the sub-scanning direction, β be the e⁻²diameter, in the main scanning direction, of the beam/p, α be the e⁻²diameter, in the sub-scanning direction, of the beam/p, ζ be the halfexposure amount of the photosensitive body/average exposure energy, andη be the intensity of one beam relative to the peak intensity of theother beam at the middle point of a line connecting the centers of twoneighboring beams. Then, the following relations hold: ##EQU11##Therefore, image omission or toner attachment to a non-exposed portionowing to the influence of interference upon irradiating N_(i) laserbeams onto proximate positions on the image surface can be prevented.Also, since the influence of the phase difference between adjacent onesof N_(i) laser beams need not be taken into consideration, the cost ofthe unit can be reduced.

According to the light scanning unit of the present invention, since Msets of optical members assigned positive power in only the sub-scanningdirection are constituted by a one-sided cylinder lens consisting ofglass and a double-sided cylinder lens substantially equivalent to thematerial of a post-deflection optical system lens, the degree of freedomin materials that can be used as a glass lens can be increased.Therefore, the cost of the light scanning unit can be reduced.

Furthermore, according to the light scanning unit of the presentinvention, since the number and reflectances of synthesis means forsynthesizing beams passing through M lenses indicated by M=1 to M=j andlocated at the side close to the light sources are optimized, theoutputs from the light sources for M can be set nearly equal to eachother. Therefore, the cost of the laser elements, laser driving units,and synthesis means can be reduced.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

We claim:
 1. An optical exposure unit comprising:M sets of light sourcesfor emitting N_(i) light beams, wherein M is an integer not less than 1,i=1 to M, and at least one set of the light sources that satisfies i≧2;first lens means for converting the light beams emitted by each of thelight sources into one of convergent light and collimated light, saidfirst lens means including one of a finite lens and collimate lens innumber corresponding to a sum of N₁ to N_(M) ; second lens means givenlens power associated with a first direction to converge the light beamsoutput from each of said first lens means in only the first direction, Msets of said second lens means being provided; M-1 synthesizingreflection mirrors that reflect M-1 groups of beams from M-1 sets ofsaid second lens means to be substantially overlaid as M groups of beamsfrom M sets of said second lens means in the first direction; deflectionmeans for deflecting the light beams output from said second lens meansand said reflection mirrors in a second direction perpendicular to thefirst direction, the deflection means including a reflection surfaceformed to be rotatable about a rotation axis extending in a directionparallel to the first direction as a center of rotation; and imagingmeans, including at least one lens, for imaging each of the light beamsdeflected at an equal speed by said deflection means at a predeterminedposition, wherein an effective field angle φ of a beam deflected by saiddeflection means satisfies:

    1.2 (W/L.sub.t)>φ>W/L.sub.t

where L_(t) is a distance between a reflection point on said deflectionmeans and an image surface, andW is an effective image region widthincluding a region where a horizontal synchronization signal isdetected.
 2. An optical exposure unit comprising:M sets of light sourcesfor emitting N_(i) light beams, wherein M is an integer not less than 1,i=1 to M, and at least one set of the light sources that satisfies i≧2;a plurality of first lenses that convert the light beams emitted by eachof the light sources into one of convergent light and collimated light,said plurality of first lenses means including one of a finite lens andcollimate lens in number corresponding to a sum of N₁ to N_(M) ; aplurality of second lenses having a lens power associated with a firstdirection to converge the light beams output from each of said firstlenses only in the first direction, wherein M sets of said second lensesare provided; M-1 synthesizing reflection mirrors that reflect M-1groups of beams from M-1 sets of said plurality of second lenses to besubstantially overlaid as M groups of beams from M sets of saidplurality of second lenses in the first direction; a deflector thatdeflects the light beams output from said plurality of second lenses andsaid reflection mirrors in a second direction perpendicular to the firstdirection, said deflector including a reflection surface rotatable abouta rotation axis that extends in a direction parallel to the firstdirection; and an imager, which includes at least one lens, that imageseach of the light beams deflected at an equal speed by said deflector ata predetermined position, wherein an effective field angle φ of a beamdeflected by said deflector satisfies:

    1.2(W/L.sub.t)>φ>W/L.sub.t

where L_(t) is a distance between a reflection point on said deflectorand an image surface, andW is an effective image region width includinga region where a horizontal synchronization signal is detected.